diff --git a/serversession-backend-acid-state/serversession-backend-acid-state.cabal b/serversession-backend-acid-state/serversession-backend-acid-state.cabal
index f32ffea..f32d874 100644
--- a/serversession-backend-acid-state/serversession-backend-acid-state.cabal
+++ b/serversession-backend-acid-state/serversession-backend-acid-state.cabal
@@ -21,6 +21,7 @@ library
     , containers
     , mtl
     , safecopy                  == 0.8.*
+    , unordered-containers
 
     , serversession             == 1.0.*
   exposed-modules:
@@ -40,7 +41,7 @@ test-suite tests
   type: exitcode-stdio-1.0
   hs-source-dirs:  tests
   build-depends:
-      base, acid-state, containers, mtl, safecopy
+      base, acid-state, containers, mtl, safecopy, unordered-containers
 
     , hspec                     >= 2.1 && < 3
 
diff --git a/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs b/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
index bc0f452..b033a5b 100644
--- a/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
+++ b/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
@@ -33,6 +33,7 @@ import Data.SafeCopy
 import Data.Typeable (Typeable)
 
 import qualified Control.Exception as E
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Map.Strict as M
 import qualified Data.Set as S
 import qualified Web.ServerSession.Core as SS
@@ -100,7 +101,11 @@ nothingfy s = if S.null s then Nothing else Just s
 ----------------------------------------------------------------------
 
 
-deriveSafeCopy 0 'base ''SS.SessionMap
+-- | We can't @deriveSafeCopy 0 'base ''SS.SessionMap@ because
+-- @safeCopy@ doesn't contain instances for @HashMap@ as of now.
+instance SafeCopy SS.SessionMap where
+  putCopy = contain . safePut . HM.toList . SS.unSessionMap
+  getCopy = contain $ SS.SessionMap . HM.fromList <$> safeGet
 
 
 -- | We can't @deriveSafeCopy 0 'base ''SS.SessionId@ as
diff --git a/serversession-backend-persistent/serversession-backend-persistent.cabal b/serversession-backend-persistent/serversession-backend-persistent.cabal
index fa026c9..18b2f9d 100644
--- a/serversession-backend-persistent/serversession-backend-persistent.cabal
+++ b/serversession-backend-persistent/serversession-backend-persistent.cabal
@@ -21,13 +21,13 @@ library
     , base64-bytestring         == 1.0.*
     , bytestring
     , cereal                    >= 0.4
-    , containers
     , path-pieces
     , persistent                == 2.1.*
     , tagged                    >= 0.8
     , text
     , time
     , transformers
+    , unordered-containers
 
     , serversession             == 1.0.*
   exposed-modules:
@@ -58,9 +58,9 @@ test-suite tests
   hs-source-dirs:  tests
   build-depends:
 
-      base, aeson, base64-bytestring, bytestring, cereal, containers,
+      base, aeson, base64-bytestring, bytestring, cereal,
       path-pieces, persistent, persistent-template, text, time,
-      transformers
+      transformers, unordered-containers
 
     , hspec                     >= 2.1 && < 3
     , monad-logger
diff --git a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
index e3e79b5..ec02314 100644
--- a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
+++ b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
@@ -25,7 +25,6 @@ import Web.ServerSession.Core
 
 import qualified Control.Exception as E
 import qualified Data.Aeson as A
-import qualified Data.Map as M
 import qualified Data.Text as T
 import qualified Database.Persist as P
 import qualified Database.Persist.Sql as P
@@ -95,7 +94,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         []
         []
         ["Eq", "Ord", "Show", "Typeable"]
-        M.empty
+        mempty
         False
     where
       pfd :: P.EntityField (PersistentSession sess) typ -> P.FieldDef
diff --git a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs
index 74d6f03..b54b396 100644
--- a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs
+++ b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs
@@ -25,7 +25,7 @@ import Web.ServerSession.Core.Internal (SessionId(..))
 
 import qualified Data.Aeson as A
 import qualified Data.ByteString.Base64.URL as B64URL
-import qualified Data.Map as M
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Serialize as S
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as TE
@@ -102,17 +102,17 @@ instance PersistFieldSql SessionMap where
   sqlType _ = SqlBlob
 
 instance S.Serialize SessionMap where
-  put = S.put . map (first TE.encodeUtf8) . M.toAscList . unSessionMap
-  get = SessionMap . M.fromAscList . map (first TE.decodeUtf8) <$> S.get
+  put = S.put . map (first TE.encodeUtf8) . HM.toList . unSessionMap
+  get = SessionMap . HM.fromList . map (first TE.decodeUtf8) <$> S.get
 
 instance A.FromJSON SessionMap where
   parseJSON = fmap fixup . A.parseJSON
     where
-      fixup :: M.Map Text ByteStringJ -> SessionMap
+      fixup :: HM.HashMap Text ByteStringJ -> SessionMap
       fixup = SessionMap . fmap unB
 
 instance A.ToJSON SessionMap where
   toJSON = A.toJSON . mangle
     where
-      mangle :: SessionMap -> M.Map Text ByteStringJ
+      mangle :: SessionMap -> HM.HashMap Text ByteStringJ
       mangle = fmap B . unSessionMap
diff --git a/serversession-backend-redis/serversession-backend-redis.cabal b/serversession-backend-redis/serversession-backend-redis.cabal
index 1c1dbf2..98b3264 100644
--- a/serversession-backend-redis/serversession-backend-redis.cabal
+++ b/serversession-backend-redis/serversession-backend-redis.cabal
@@ -18,13 +18,13 @@ library
   build-depends:
       base                      == 4.*
     , bytestring
-    , containers
     , hedis                     == 0.6.*
     , path-pieces
     , tagged                    >= 0.8
     , text
     , time                      >= 1.5
     , transformers
+    , unordered-containers
 
     , serversession             == 1.0.*
   exposed-modules:
@@ -44,8 +44,8 @@ test-suite tests
   type: exitcode-stdio-1.0
   hs-source-dirs:  tests
   build-depends:
-      base, bytestring, containers, hedis, path-pieces, text,
-      time, transformers
+      base, bytestring, hedis, path-pieces, text,
+      time, transformers, unordered-containers
 
     , hspec                     >= 2.1 && < 3
 
diff --git a/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs b/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
index 261750f..49a33a8 100644
--- a/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
+++ b/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
@@ -42,7 +42,7 @@ import qualified Control.Exception as E
 import qualified Database.Redis as R
 import qualified Data.ByteString as B
 import qualified Data.ByteString.Char8 as B8
-import qualified Data.Map.Strict as M
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Text.Encoding as TE
 import qualified Data.Time.Clock as TI
 import qualified Data.Time.Format as TI
@@ -141,8 +141,8 @@ class IsSessionData sess => RedisSession sess where
 -- | Assumes that keys are UTF-8 encoded when parsing (which is
 -- true if keys are always generated via @toHash@).
 instance RedisSession SessionMap where
-  toHash   _ = map (first TE.encodeUtf8) . M.toList . unSessionMap
-  fromHash _ = SessionMap . M.fromList . map (first TE.decodeUtf8)
+  toHash   _ = map (first TE.encodeUtf8) . HM.toList . unSessionMap
+  fromHash _ = SessionMap . HM.fromList . map (first TE.decodeUtf8)
 
 
 -- | Parse a 'Session' from a Redis hash.
diff --git a/serversession-frontend-snap/serversession-frontend-snap.cabal b/serversession-frontend-snap/serversession-frontend-snap.cabal
index 7e4d670..92fd32c 100644
--- a/serversession-frontend-snap/serversession-frontend-snap.cabal
+++ b/serversession-frontend-snap/serversession-frontend-snap.cabal
@@ -18,7 +18,6 @@ library
   build-depends:
       base                      == 4.*
     , bytestring
-    , containers
     , nonce
     , path-pieces
     , snap                      == 0.14.*
@@ -26,6 +25,7 @@ library
     , text
     , time
     , transformers
+    , unordered-containers
 
     , serversession             == 1.0.*
   exposed-modules:
diff --git a/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs b/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs
index 3daeeec..84c1874 100644
--- a/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs
+++ b/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs
@@ -24,10 +24,10 @@ import Web.ServerSession.Core
 
 import qualified Crypto.Nonce as N
 import qualified Data.ByteString.Char8 as B8
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as TE
 import qualified Data.Time as TI
-import qualified Data.Map as M
 import qualified Snap.Core as S
 import qualified Snap.Snaplet as S
 import qualified Snap.Snaplet.Session as S
@@ -84,27 +84,27 @@ class IsSessionData sess => SnapSession sess where
 
 -- | Uses 'csrfKey'.
 instance SnapSession SessionMap where
-  ssInsert key val = onSM (M.insert key (TE.encodeUtf8 val))
-  ssLookup key     = fmap TE.decodeUtf8 . M.lookup key . unSessionMap
-  ssDelete key     = onSM (M.delete key)
+  ssInsert key val = onSM (HM.insert key (TE.encodeUtf8 val))
+  ssLookup key     = fmap TE.decodeUtf8 . HM.lookup key . unSessionMap
+  ssDelete key     = onSM (HM.delete key)
   ssToList =
     -- Remove the CSRF key from the list as the current
     -- clientsession backend doesn't return it.
     fmap (second TE.decodeUtf8) .
-    M.toList .
-    M.delete csrfKey .
+    HM.toList .
+    HM.delete csrfKey .
     unSessionMap
 
   ssInsertCsrf = ssInsert csrfKey
   ssLookupCsrf = ssLookup csrfKey
 
-  ssForceInvalidate force = onSM (M.insert forceInvalidateKey (B8.pack $ show force))
+  ssForceInvalidate force = onSM (HM.insert forceInvalidateKey (B8.pack $ show force))
 
 
 -- | Apply a function to a 'SessionMap'.
 onSM
-  :: (M.Map Text ByteString -> M.Map Text ByteString)
-  -> (SessionMap            -> SessionMap)
+  :: (HM.HashMap Text ByteString -> HM.HashMap Text ByteString)
+  -> (SessionMap                 -> SessionMap)
 onSM f = SessionMap . f . unSessionMap
 
 
diff --git a/serversession-frontend-wai/serversession-frontend-wai.cabal b/serversession-frontend-wai/serversession-frontend-wai.cabal
index c03d55c..5889446 100644
--- a/serversession-frontend-wai/serversession-frontend-wai.cabal
+++ b/serversession-frontend-wai/serversession-frontend-wai.cabal
@@ -18,13 +18,13 @@ library
   build-depends:
       base                      >= 4.6   && < 5
     , bytestring
-    , containers
     , cookie                    >= 0.4
     , data-default
     , path-pieces
     , text
     , time
     , transformers
+    , unordered-containers
     , vault
     , wai
     , wai-session               == 0.3.*
diff --git a/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs b/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs
index 36f93ac..320d26c 100644
--- a/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs
+++ b/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs
@@ -21,8 +21,8 @@ import Web.ServerSession.Core
 import Web.ServerSession.Core.Internal (absoluteTimeout, idleTimeout, persistentCookies)
 
 import qualified Data.ByteString.Char8 as B8
+import qualified Data.HashMap.Strict as HM
 import qualified Data.IORef as I
-import qualified Data.Map as M
 import qualified Data.Text.Encoding as TE
 import qualified Data.Time as TI
 import qualified Data.Vault.Lazy as V
@@ -100,8 +100,8 @@ class IsSessionData sess => KeyValue sess where
 instance KeyValue SessionMap where
   type Key   SessionMap = Text
   type Value SessionMap = ByteString
-  kvLookup k = M.lookup k . unSessionMap
-  kvInsert k v (SessionMap m) = SessionMap (M.insert k v m)
+  kvLookup k = HM.lookup k . unSessionMap
+  kvInsert k v (SessionMap m) = SessionMap (HM.insert k v m)
 
 
 ----------------------------------------------------------------------
diff --git a/serversession-frontend-yesod/serversession-frontend-yesod.cabal b/serversession-frontend-yesod/serversession-frontend-yesod.cabal
index c22c26e..95becb7 100644
--- a/serversession-frontend-yesod/serversession-frontend-yesod.cabal
+++ b/serversession-frontend-yesod/serversession-frontend-yesod.cabal
@@ -25,6 +25,7 @@ library
     , text
     , time
     , transformers
+    , unordered-containers
     , wai
     , yesod-core                == 1.4.*
 
diff --git a/serversession-frontend-yesod/src/Web/ServerSession/Frontend/Yesod/Internal.hs b/serversession-frontend-yesod/src/Web/ServerSession/Frontend/Yesod/Internal.hs
index cd5144e..b913b57 100644
--- a/serversession-frontend-yesod/src/Web/ServerSession/Frontend/Yesod/Internal.hs
+++ b/serversession-frontend-yesod/src/Web/ServerSession/Frontend/Yesod/Internal.hs
@@ -21,6 +21,7 @@ import Yesod.Core.Handler (setSessionBS)
 import Yesod.Core.Types (Header(AddCookie), SessionBackend(..))
 
 import qualified Data.ByteString.Char8 as B8
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Map as M
 import qualified Data.Text.Encoding as TE
 import qualified Data.Time as TI
@@ -107,8 +108,8 @@ class IsSessionMap sess where
 
 
 instance IsSessionMap SessionMap where
-  toSessionMap   = unSessionMap
-  fromSessionMap =   SessionMap
+  toSessionMap   = M.fromList . HM.toList . unSessionMap
+  fromSessionMap = SessionMap . HM.fromList . M.toList
 
 
 ----------------------------------------------------------------------
diff --git a/serversession/serversession.cabal b/serversession/serversession.cabal
index 8c9618e..535df3d 100644
--- a/serversession/serversession.cabal
+++ b/serversession/serversession.cabal
@@ -20,13 +20,14 @@ library
     , aeson
     , base64-bytestring         == 1.0.*
     , bytestring
-    , containers
     , data-default
+    , hashable
     , nonce                     == 1.0.*
     , path-pieces
     , text
     , time
     , transformers
+    , unordered-containers
   exposed-modules:
     Web.ServerSession.Core
     Web.ServerSession.Core.Internal
@@ -47,9 +48,11 @@ test-suite tests
   type: exitcode-stdio-1.0
   hs-source-dirs:  tests
   build-depends:
-      base, aeson, base64-bytestring, bytestring, containers,
-      data-default, nonce, path-pieces, text, time, transformers
+      base, aeson, base64-bytestring, bytestring, data-default,
+      nonce, path-pieces, text, time, transformers,
+      unordered-containers
 
+    , containers
     , hspec                     >= 2.1 && < 3
     , QuickCheck
     , serversession
diff --git a/serversession/src/Web/ServerSession/Core/Internal.hs b/serversession/src/Web/ServerSession/Core/Internal.hs
index 30bed40..26b4f13 100644
--- a/serversession/src/Web/ServerSession/Core/Internal.hs
+++ b/serversession/src/Web/ServerSession/Core/Internal.hs
@@ -49,6 +49,7 @@ import Control.Applicative ((<$>), (<*>))
 import Control.Monad (guard, when)
 import Control.Monad.IO.Class (MonadIO(..))
 import Data.ByteString (ByteString)
+import Data.Hashable (Hashable(..))
 import Data.Maybe (catMaybes, fromMaybe, isJust)
 import Data.Text (Text)
 import Data.Time (UTCTime, getCurrentTime)
@@ -61,7 +62,7 @@ import qualified Crypto.Nonce as N
 import qualified Data.Aeson as A
 import qualified Data.ByteString.Base64.URL as B64URL
 import qualified Data.ByteString.Char8 as B8
-import qualified Data.Map as M
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as TE
 
@@ -93,6 +94,9 @@ instance A.FromJSON (SessionId sess) where
 instance A.ToJSON (SessionId sess) where
   toJSON = A.toJSON . unS
 
+instance Hashable (SessionId sess) where
+  hashWithSalt s = hashWithSalt s . unS
+
 
 -- | (Internal) Check that the given text is a base64url-encoded
 -- representation of 18 bytes.
@@ -150,8 +154,8 @@ deriving instance Show (Decomposed sess) => Show (Session sess)
 -- to support this session data type on all frontends and storage
 -- backends.
 newtype SessionMap =
-  SessionMap { unSessionMap :: M.Map Text ByteString }
-  deriving (Eq, Ord, Show, Read, Typeable)
+  SessionMap { unSessionMap :: HM.HashMap Text ByteString }
+  deriving (Eq, Show, Read, Typeable)
 
 
 ----------------------------------------------------------------------
@@ -224,22 +228,25 @@ class ( Show (Decomposed sess)
 instance IsSessionData SessionMap where
   type Decomposed SessionMap = SessionMap
 
-  emptySession = SessionMap M.empty
+  emptySession = SessionMap HM.empty
 
   isSameDecomposed _ = (==)
 
   decomposeSession authKey_ (SessionMap sm1) =
-    let (authId, sm2) = M.updateLookupWithKey (\_ _ -> Nothing) authKey_           sm1
-        (force,  sm3) = M.updateLookupWithKey (\_ _ -> Nothing) forceInvalidateKey sm2
+    let authId = HM.lookup authKey_ sm1
+        force  = maybe DoNotForceInvalidate (read . B8.unpack) $
+                 HM.lookup forceInvalidateKey sm1
+        sm2    = HM.delete authKey_ $
+                 HM.delete forceInvalidateKey sm1
     in DecomposedSession
          { dsAuthId          = authId
-         , dsForceInvalidate = maybe DoNotForceInvalidate (read . B8.unpack) force
-         , dsDecomposed      = SessionMap sm3 }
+         , dsForceInvalidate = force
+         , dsDecomposed      = SessionMap sm2 }
 
   recomposeSession authKey_ mauthId (SessionMap sm) =
-    SessionMap $ maybe id (M.insert authKey_) mauthId sm
+    SessionMap $ maybe id (HM.insert authKey_) mauthId sm
 
-  isDecomposedEmpty _ = M.null . unSessionMap
+  isDecomposedEmpty _ = HM.null . unSessionMap
 
 
 -- | A session data type @sess@ with its special variables taken apart.
diff --git a/serversession/src/Web/ServerSession/Core/StorageTests.hs b/serversession/src/Web/ServerSession/Core/StorageTests.hs
index 1f08f5c..3591a5c 100644
--- a/serversession/src/Web/ServerSession/Core/StorageTests.hs
+++ b/serversession/src/Web/ServerSession/Core/StorageTests.hs
@@ -14,7 +14,7 @@ import Web.ServerSession.Core.Internal
 
 import qualified Crypto.Nonce as N
 import qualified Data.ByteString as B
-import qualified Data.Map as M
+import qualified Data.HashMap.Strict as HM
 import qualified Data.Text as T
 import qualified Data.Time as TI
 
@@ -172,11 +172,11 @@ allStorageTests storage it runIO parallel _shouldBe shouldReturn shouldThrow = d
         let session = Session
               { sessionKey        = sid
               , sessionAuthId     = Nothing
-              , sessionData       = SessionMap $ M.fromList vals
+              , sessionData       = SessionMap $ HM.fromList vals
               , sessionCreatedAt  = now
               , sessionAccessedAt = now
               }
-            ver2 = session { sessionData = SessionMap M.empty }
+            ver2 = session { sessionData = SessionMap HM.empty }
         run (getSession storage sid) `shouldReturn` Nothing
         run (insertSession storage session)
         run (getSession storage sid) `shouldReturn` (Just session)
@@ -217,7 +217,7 @@ generateSession gen hasAuthId = do
   data_ <- do
     keys <- replicateM 20 (N.nonce128urlT gen)
     vals <- replicateM 20 (N.nonce128url  gen)
-    return $ M.fromList (zip keys vals)
+    return $ HM.fromList (zip keys vals)
   now <- TI.getCurrentTime
   return Session
     { sessionKey        = sid
diff --git a/serversession/tests/Main.hs b/serversession/tests/Main.hs
index fe47403..2188a31 100644
--- a/serversession/tests/Main.hs
+++ b/serversession/tests/Main.hs
@@ -14,8 +14,8 @@ import Web.ServerSession.Core.StorageTests
 import qualified Control.Exception as E
 import qualified Crypto.Nonce as N
 import qualified Data.ByteString.Char8 as B8
+import qualified Data.HashMap.Strict as HM
 import qualified Data.IORef as I
-import qualified Data.Map as M
 import qualified Data.Set as S
 import qualified Data.Text as T
 import qualified Data.Time as TI
@@ -128,7 +128,7 @@ main = hspec $ parallel $ do
       st  <- createState =<< prepareMockStorage [session]
       (retSessionMap, SaveSessionToken msession _now) <-
           loadSession st (Just $ B8.pack $ T.unpack $ unS $ sessionKey session)
-      retSessionMap `shouldBe` onSM (M.insert (authKey st) authId) (sessionData session)
+      retSessionMap `shouldBe` onSM (HM.insert (authKey st) authId) (sessionData session)
       msession      `shouldBe` Just session
 
   describe "checkExpired" $ do
@@ -224,22 +224,22 @@ main = hspec $ parallel $ do
       sessionData   session1 `shouldBe` m1
       getMockOperations sto `shouldReturn` [InsertSession session1]
 
-      let m2 = onSM (M.insert (authKey st) "john") m1
+      let m2 = onSM (HM.insert (authKey st) "john") m1
       Just session2 <- saveSession st (SaveSessionToken (Just session1) fakenow) m2
       sessionAuthId session2 `shouldBe` Just "john"
       sessionData   session2 `shouldBe` m1
       sessionKey session2 == sessionKey session1 `shouldBe` False
       getMockOperations sto `shouldReturn` [DeleteSession (sessionKey session1), InsertSession session2]
 
-      let m3 = onSM (M.insert forceInvalidateKey (B8.pack $ show AllSessionIdsOfLoggedUser)) m2
+      let m3 = onSM (HM.insert forceInvalidateKey (B8.pack $ show AllSessionIdsOfLoggedUser)) m2
       Just session3 <- saveSession st (SaveSessionToken (Just session2) fakenow) m3
       session3 `shouldBe` session2 { sessionKey = sessionKey session3 }
       getMockOperations sto `shouldReturn`
         [DeleteSession (sessionKey session2), DeleteAllSessionsOfAuthId "john", InsertSession session3]
 
-      let m4 = onSM (M.insert "x" "y") m2
+      let m4 = onSM (HM.insert "x" "y") m2
       Just session4 <- saveSession st (SaveSessionToken (Just session3) fakenow) m4
-      session4 `shouldBe` session3 { sessionData = onSM (M.delete (authKey st)) m4 }
+      session4 `shouldBe` session3 { sessionData = onSM (HM.delete (authKey st)) m4 }
       getMockOperations sto `shouldReturn` [ReplaceSession session4]
 
       Just session5 <- saveSession st (SaveSessionToken (Just session4) (TI.addUTCTime 10 fakenow)) m4
@@ -369,13 +369,13 @@ main = hspec $ parallel $ do
 
     prop "parses the force invalidate key" $
       \data_  ->
-        let sessionMap v = onSM (M.insert forceInvalidateKey (B8.pack $ show v)) $ mkSessionMap data_
+        let sessionMap v = onSM (HM.insert forceInvalidateKey (B8.pack $ show v)) $ mkSessionMap data_
             allForces    = [minBound..maxBound] :: [ForceInvalidate]
             test v       = dsForceInvalidate (decomposeSession authKey_ $ sessionMap v) Q.=== v
         in Q.conjoin (test <$> allForces)
 
     it "removes the auth key" $ do
-      let m = M.singleton "a" "b"; m' = M.insert (authKey stnull) "x" m
+      let m = HM.singleton "a" "b"; m' = HM.insert (authKey stnull) "x" m
       decomposeSession authKey_ (SessionMap m') `shouldBe`
         DecomposedSession (Just "x") DoNotForceInvalidate (SessionMap m)
 
@@ -392,7 +392,7 @@ main = hspec $ parallel $ do
         let s = mkSessionMap ((T.unpack authKey_, "foo") : data_)
             authId = B8.pack authId_
         in       recomposeSession authKey_ (Just authId) s
-           Q.=== onSM (M.adjust (const authId) authKey_) s
+           Q.=== onSM (HM.adjust (const authId) authKey_) s
 
   describe "MockStorage" $ do
     sto <- runIO emptyMockStorage
@@ -401,13 +401,13 @@ main = hspec $ parallel $ do
 
 -- | Used to generate session maps on QuickCheck properties.
 mkSessionMap :: [(String, String)] -> SessionMap
-mkSessionMap = SessionMap . M.fromList . map (T.pack *** B8.pack)
+mkSessionMap = SessionMap . HM.fromList . map (T.pack *** B8.pack)
 
 
 -- | Apply a function to a 'SessionMap'.
 onSM
-  :: (M.Map T.Text B8.ByteString -> M.Map T.Text B8.ByteString)
-  -> (SessionMap                 -> SessionMap)
+  :: (HM.HashMap T.Text B8.ByteString -> HM.HashMap T.Text B8.ByteString)
+  -> (SessionMap                      -> SessionMap)
 onSM f = SessionMap . f . unSessionMap
 
 
@@ -484,7 +484,7 @@ deriving instance Show (Decomposed sess) => Show (MockOperation sess)
 -- | A mock storage used just for testing.
 data MockStorage sess =
   MockStorage
-    { mockSessions   :: I.IORef (M.Map (SessionId sess) (Session sess))
+    { mockSessions   :: I.IORef (HM.HashMap (SessionId sess) (Session sess))
     , mockOperations :: I.IORef [MockOperation sess]
     }
   deriving (Typeable)
@@ -498,30 +498,24 @@ instance IsSessionData sess => Storage (MockStorage sess) where
     -- because latter may be reordered (cf. "Memory Model" on
     -- Data.IORef's documentation).
     addMockOperation sto (GetSession sid)
-    M.lookup sid <$> I.atomicModifyIORef' (mockSessions sto) (\a -> (a, a))
+    HM.lookup sid <$> I.atomicModifyIORef' (mockSessions sto) (\a -> (a, a))
   deleteSession sto sid = do
-    I.atomicModifyIORef' (mockSessions sto) ((, ()) . M.delete sid)
+    I.atomicModifyIORef' (mockSessions sto) ((, ()) . HM.delete sid)
     addMockOperation sto (DeleteSession sid)
   deleteAllSessionsOfAuthId sto authId = do
-    I.atomicModifyIORef' (mockSessions sto) ((, ()) . M.filter (\s -> sessionAuthId s /= Just authId))
+    I.atomicModifyIORef' (mockSessions sto) ((, ()) . HM.filter (\s -> sessionAuthId s /= Just authId))
     addMockOperation sto (DeleteAllSessionsOfAuthId authId)
   insertSession sto session = do
     join $ I.atomicModifyIORef' (mockSessions sto) $ \oldMap ->
-      let (moldVal, newMap) =
-            M.insertLookupWithKey (\_ v _ -> v) (sessionKey session) session oldMap
-      in maybe
-           (newMap, return ())
-           (\oldVal -> (oldMap, mockThrow $ SessionAlreadyExists oldVal session))
-           moldVal
+      case HM.lookup (sessionKey session) oldMap of
+        Just oldVal -> (oldMap, mockThrow $ SessionAlreadyExists oldVal session)
+        Nothing     -> (HM.insert (sessionKey session) session oldMap, return ())
     addMockOperation sto (InsertSession session)
   replaceSession sto session = do
     join $ I.atomicModifyIORef' (mockSessions sto) $ \oldMap ->
-      let (moldVal, newMap) =
-            M.insertLookupWithKey (\_ v _ -> v) (sessionKey session) session oldMap
-      in maybe
-           (oldMap, mockThrow $ SessionDoesNotExist session)
-           (const (newMap, return ()))
-           moldVal
+      case HM.lookup (sessionKey session) oldMap of
+        Just _  -> (HM.insert (sessionKey session) session oldMap, return ())
+        Nothing -> (oldMap, mockThrow $ SessionDoesNotExist session)
     addMockOperation sto (ReplaceSession session)
 
 
@@ -537,7 +531,7 @@ mockThrow = E.throwIO
 emptyMockStorage :: IO (MockStorage sess)
 emptyMockStorage =
   MockStorage
-    <$> I.newIORef M.empty
+    <$> I.newIORef HM.empty
     <*> I.newIORef []
 
 
@@ -545,7 +539,7 @@ emptyMockStorage =
 prepareMockStorage :: [Session sess] -> IO (MockStorage sess)
 prepareMockStorage sessions = do
   sto <- emptyMockStorage
-  I.writeIORef (mockSessions sto) (M.fromList [(sessionKey s, s) | s <- sessions])
+  I.writeIORef (mockSessions sto) (HM.fromList [(sessionKey s, s) | s <- sessions])
   return sto