diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,11 @@
 = HEAD =
 
+= 0.9.1.0 =
+
+ - Updated dependency versions (cereal, specifically) to account for ghc-8.0.
+ - Switched to using HashSet in VectorSim; gaining ~25% performance improvement (Thanks to @dgaw!)
+ - Dropped support for ghc-7.6.
+
 = 0.9.0.0 =
 
  - Regenerated the model files; changes to cereal caused the old
diff --git a/chatter.cabal b/chatter.cabal
--- a/chatter.cabal
+++ b/chatter.cabal
@@ -1,5 +1,5 @@
 name:                chatter
-version:             0.9.0.0
+version:             0.9.1.0
 synopsis:            A library of simple NLP algorithms.
 description:         chatter is a collection of simple Natural Language
                      Processing algorithms.
@@ -77,7 +77,7 @@
                      containers     >= 0.5.0.0,
                      random-shuffle >= 0.0.4,
                      MonadRandom    >= 0.1.2,
-                     cereal         >= 0.4.0.1 && < 0.5.3.0,
+                     cereal         >= 0.4.0.1 && < 0.5.4.0,
                      cereal-text    >= 0.1 && < 0.2,
                      fullstop       >= 0.1.3.1,
                      bytestring     >= 0.10.0.0,
@@ -112,7 +112,7 @@
                      text >= 0.11.3.0,
                      base >= 4.6 && <= 6,
                      bytestring >= 0.10.0.0,
-                     cereal >= 0.4.0.1 && < 0.5.3.0
+                     cereal >= 0.4.0.1 && < 0.5.4.0
 
    ghc-options:      -Wall -main-is Tagger -rtsopts
 
@@ -126,7 +126,7 @@
                      text >= 0.11.3.0,
                      base >= 4.6 && <= 6,
                      bytestring >= 0.10.0.0,
-                     cereal >= 0.4.0.1 && < 0.5.3.0,
+                     cereal >= 0.4.0.1 && < 0.5.4.0,
                      containers >= 0.5.0.0
 
    ghc-options:      -Wall -main-is POSTrainer -rtsopts
@@ -141,7 +141,7 @@
                      text >= 0.11.3.0,
                      base >= 4.6 && <= 6,
                      bytestring >= 0.10.0.0,
-                     cereal >= 0.4.0.1 && < 0.5.3.0,
+                     cereal >= 0.4.0.1 && < 0.5.4.0,
                      containers >= 0.5.0.0
 
    ghc-options:      -Wall -main-is ChunkTrainer -rtsopts
@@ -156,7 +156,7 @@
                      text >= 0.11.3.0,
                      base >= 4.6 && <= 6,
                      bytestring >= 0.10.0.0,
-                     cereal >= 0.4.0.1 && < 0.5.3.0,
+                     cereal >= 0.4.0.1 && < 0.5.4.0,
                      containers >= 0.5.0.0
 
    ghc-options:      -Wall -main-is NERTrainer -rtsopts
@@ -172,7 +172,7 @@
                      text >= 0.11.3.0,
                      base >= 4.6 && <= 6,
                      bytestring >= 0.10.0.0,
-                     cereal >= 0.4.0.1 && < 0.5.3.0,
+                     cereal >= 0.4.0.1 && < 0.5.4.0,
                      containers >= 0.5.0.0
 
    ghc-options:      -Wall -main-is Evaluate -rtsopts
@@ -226,7 +226,7 @@
                      tokenize,
                      QuickCheck,
                      filepath,
-                     cereal >= 0.4.0.1 && < 0.5.3.0,
+                     cereal >= 0.4.0.1 && < 0.5.4.0,
                      quickcheck-instances,
                      containers,
                      tasty,
diff --git a/src/NLP/Similarity/VectorSim.hs b/src/NLP/Similarity/VectorSim.hs
--- a/src/NLP/Similarity/VectorSim.hs
+++ b/src/NLP/Similarity/VectorSim.hs
@@ -8,7 +8,7 @@
 import Test.QuickCheck (Arbitrary(..))
 import qualified Data.HashMap.Strict as HM
 import qualified Data.DefaultMap as DM
-import qualified Data.Set as Set
+import qualified Data.HashSet as HSet
 import Data.Text (Text)
 import qualified Data.Text as T
 import Data.List (elemIndices)
@@ -72,7 +72,7 @@
 -- implementation.  If you need to run similarity against any single
 -- document more than once, then you should create `TermVector`s for
 -- each of your documents and use `tvSim` instead of `similarity`.
--- 
+--
 -- The return value will be in the range [0, 1].
 --
 -- There *must* be at least one document in the corpus.
@@ -153,12 +153,16 @@
 -- | find the dot product of two vectors.
 dotProd :: TermVector -> TermVector -> Double
 dotProd xs ys = let
-  terms = Set.fromList (keys xs) `Set.union` Set.fromList (keys ys)
-  in Set.foldl (+) 0 (Set.map (\t -> (lookup t xs) * (lookup t ys)) terms)
+  -- Perhaps a bit more performance can be had if we ensure the first set
+  -- in the union is the smaller one?
+  -- https://hackage.haskell.org/package/unordered-containers-0.2.7.2/docs/Data-HashSet.html#v:union
+  terms = vectorToSet xs `HSet.union` vectorToSet ys
+  in HSet.foldl' (+) 0 (HSet.map (\t -> (lookup t xs) * (lookup t ys)) terms)
+  where
+    vectorToSet = HSet.fromMap . (HM.map (const ())) . DM.defMap . fromTV
 
 keys :: TermVector -> [Text]
 keys tv = DM.keys $ fromTV tv
 
 lookup :: Text -> TermVector -> Double
 lookup key tv = DM.lookup key $ fromTV tv
-