diff --git a/src/Category.hs b/src/Category.hs
index 0dc96f9b5..240ef4fee 100644
--- a/src/Category.hs
+++ b/src/Category.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE DeriveAnyClass #-}
 {-# OPTIONS_GHC -funbox-strict-fields #-}
 module Category where
 
@@ -227,7 +228,7 @@ data Category
   | Modifier Category
   -- | A singleton method declaration, e.g. `def self.foo;end` in Ruby
   | SingletonMethod
-  deriving (Eq, Generic, Ord, Show)
+  deriving (Eq, Generic, Ord, Show, NFData)
 
 {-# DEPRECATED RescueModifier "Deprecated; use Modifier Rescue instead." #-}
 
diff --git a/src/Data/Functor/Both.hs b/src/Data/Functor/Both.hs
index f0b78dd21..ad914f505 100644
--- a/src/Data/Functor/Both.hs
+++ b/src/Data/Functor/Both.hs
@@ -31,3 +31,5 @@ instance (Semigroup a, Monoid a) => Monoid (Join (,) a) where
 
 instance (Semigroup a) => Semigroup (Join (,) a) where
   a <> b = Join $ runJoin a <> runJoin b
+
+instance NFData a => NFData (Join (,) a)
diff --git a/src/Data/Record.hs b/src/Data/Record.hs
index bb645fb37..5e78a98fc 100644
--- a/src/Data/Record.hs
+++ b/src/Data/Record.hs
@@ -51,6 +51,11 @@ instance {-# OVERLAPPABLE #-} HasField (field ': fields) field where
   getField (h :. _) = h
   setField (_ :. t) f = f :. t
 
+instance (NFData h, NFData (Record t)) => NFData (Record (h ': t)) where
+  rnf (h :. t) = rnf h `seq` rnf t `seq` ()
+
+instance NFData (Record '[]) where
+  rnf _ = ()
 
 instance (Show h, Show (Record t)) => Show (Record (h ': t)) where
   showsPrec n (h :. t) = showParen (n > 0) $ showsPrec 1 h . (" :. " <>) . shows t
diff --git a/src/Diff.hs b/src/Diff.hs
index cc9230f46..34d3881c2 100644
--- a/src/Diff.hs
+++ b/src/Diff.hs
@@ -1,4 +1,5 @@
-{-# LANGUAGE TypeSynonymInstances, ScopedTypeVariables #-}
+{-# LANGUAGE TypeSynonymInstances, ScopedTypeVariables, UndecidableInstances #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 module Diff where
 
 import Prologue
@@ -50,3 +51,8 @@ modifyAnnotations :: (Functor f, Functor g) => (annotation -> annotation) -> Fre
 modifyAnnotations f r = case runFree r of
   Free (ga :< functor) -> wrap (fmap f ga :< functor)
   _ -> r
+
+instance (NFData (f (Diff f a)), NFData (Cofree f a), NFData a, Functor f) => NFData (Diff f a) where
+  rnf fa = case runFree fa of
+    Free f -> rnf f `seq` ()
+    Pure a -> rnf a `seq` ()
diff --git a/src/DiffCommand.hs b/src/DiffCommand.hs
index ba3578d5e..b61668159 100644
--- a/src/DiffCommand.hs
+++ b/src/DiffCommand.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE DataKinds #-}
 {-# OPTIONS_GHC -fno-warn-incomplete-patterns #-}
 module DiffCommand where
 
@@ -5,12 +6,10 @@ import Data.Aeson hiding (json)
 import Data.Functor.Both as Both
 import Data.List ((\\))
 import Data.String
-import Data.Text.Encoding (encodeUtf8)
 import GHC.Conc (numCapabilities)
 import Prologue hiding (fst, snd, null)
 import qualified Control.Concurrent.Async.Pool as Async
 import System.FilePath.Posix (hasExtension)
-import System.Timeout (timeout)
 import Git.Blob
 import Git.Libgit2
 import Git.Libgit2.Backend
@@ -22,6 +21,7 @@ import Category
 import Data.RandomWalkSimilarity
 import Data.Record
 import Info
+import Diff
 import Interpreter
 import ParseCommand (parserForFilepath)
 import Parser
@@ -35,7 +35,7 @@ import Renderer.Summary
 import Renderer.TOC
 import Source
 import Syntax
-import Term
+import Debug.Trace
 
 diff :: Arguments -> IO ByteString
 diff args@Arguments{..} = case diffMode of
@@ -45,11 +45,8 @@ diff args@Arguments{..} = case diffMode of
 -- | Compare changes between two commits.
 diffCommits :: Arguments -> IO ByteString
 diffCommits args@Arguments{..} = do
-  ts <- fetchTerms args
-  pure $ maybe mempty concatOutputs ts
-  where fetchTerms args = if developmentMode
-                            then Just <$> fetchDiffs args
-                            else timeout timeoutInMicroseconds (fetchDiffs args)
+  outputs <- fetchDiffs args
+  pure $! concatOutputs outputs
 
 -- | Compare two paths on the filesystem (compariable to git diff --no-index).
 diffPaths :: Arguments -> Both FilePath -> IO ByteString
@@ -66,40 +63,38 @@ fetchDiffs args@Arguments{..} = do
     ([], Join (Just a, Just b)) -> pathsToDiff args (both a b)
     (ps, _) -> pure ps
 
-  Async.withTaskGroup numCapabilities $ \p -> Async.mapTasks p (fetchDiff args <$> paths)
+  diffs <- Async.withTaskGroup numCapabilities . flip Async.mapTasks $
+    fetchDiff args <$> paths
+  pure $ uncurry (renderDiff args) <$> diffs
 
-fetchDiff :: Arguments -> FilePath -> IO Output
+fetchDiff :: Arguments -> FilePath -> IO (Both SourceBlob, SyntaxDiff Text '[Range, Category, SourceSpan])
 fetchDiff args@Arguments{..} filepath = withRepository lgFactory gitDir $ do
   repo <- getRepository
   for_ alternateObjectDirs (liftIO . odbBackendAddPath repo . toS)
-  lift $ runReaderT (fetchDiff' args filepath) repo
-
-fetchDiff' :: Arguments -> FilePath -> ReaderT LgRepo IO Output
-fetchDiff' args@Arguments{..} filepath = do
-  sourcesAndOids <- sequence $ traverse (getSourceBlob filepath) <$> shaRange
-
-  let sources = fromMaybe (emptySourceBlob filepath) <$> sourcesAndOids
-  let sourceBlobs = idOrEmptySourceBlob <$> sources
-
-  text <- liftIO . render $ textDiff (parserForFilepath filepath) args sourceBlobs
-  truncatedPatch <- liftIO $ truncatedDiff args sourceBlobs
-  pure $ fromMaybe truncatedPatch text
+  lift $ runReaderT (go args filepath) repo
   where
-    render output = if developmentMode
-                    then Just <$> output
-                    else timeout timeoutInMicroseconds output
+    go :: Arguments -> FilePath -> ReaderT LgRepo IO (Both SourceBlob, SyntaxDiff Text '[Range, Category, SourceSpan])
+    go Arguments{..} filepath = do
+      liftIO $ traceEventIO ("START fetchDiff: " <> filepath)
+      sourcesAndOids <- sequence $ traverse (getSourceBlob filepath) <$> shaRange
 
+      let sources = fromMaybe (emptySourceBlob filepath) <$> sourcesAndOids
+      let sourceBlobs = idOrEmptySourceBlob <$> sources
+
+      diff <- liftIO $ diffFiles (parserForFilepath filepath) sourceBlobs
+      pure $! traceEvent ("END fetchDiff: " <> filepath) (sourceBlobs, diff)
+
+-- | Returns a list of relative file paths that have changed between the given commit shas.
 pathsToDiff :: Arguments -> Both String -> IO [FilePath]
 pathsToDiff Arguments{..} shas = withRepository lgFactory gitDir $ do
   repo <- getRepository
   for_ alternateObjectDirs (liftIO . odbBackendAddPath repo . toS)
-  lift $ runReaderT (pathsToDiff' shas) repo
-
--- | Returns a list of relative file paths that have changed between the given commit shas.
-pathsToDiff' :: Both String -> ReaderT LgRepo IO [FilePath]
-pathsToDiff' shas = do
-  entries <- blobEntriesToDiff shas
-  pure $ (\(p, _, _) -> toS p) <$> entries
+  lift $ runReaderT (go shas) repo
+  where
+    go :: Both String -> ReaderT LgRepo IO [FilePath]
+    go shas = do
+      entries <- blobEntriesToDiff shas
+      pure $ (\(p, _, _) -> toS p) <$> entries
 
 -- | Returns a list of blob entries that have changed between the given commits shas.
 blobEntriesToDiff :: Both String -> ReaderT LgRepo IO [(TreeFilePath, Git.BlobOid LgRepo, BlobKind)]
@@ -136,20 +131,19 @@ getSourceBlob path sha = do
     toSourceKind (Git.ExecutableBlob mode) = Source.ExecutableBlob mode
     toSourceKind (Git.SymlinkBlob mode) = Source.SymlinkBlob mode
 
--- | Given a parser and renderer, diff two sources and return the rendered
--- | result.
--- | Returns the rendered result strictly, so it's always fully evaluated
--- | with respect to other IO actions.
-diffFiles :: HasField fields Category
+-- | Given a parser, diff two sources and return a SyntaxDiff.
+-- | Returns the rendered result strictly, so it's always fully evaluated with respect to other IO actions.
+diffFiles :: (HasField fields Category, NFData (Record fields))
           => Parser (Syntax Text) (Record fields)
-          -> Renderer (Record fields)
           -> Both SourceBlob
-          -> IO Output
-diffFiles parse render sourceBlobs = do
+          -> IO (SyntaxDiff Text fields)
+diffFiles parse sourceBlobs = do
+  traceEventIO $ "diffFiles@SEMANTIC-DIFF START parse terms: " <> paths
   terms <- Async.withTaskGroup numCapabilities . flip Async.mapTasks $
     (fmap (defaultFeatureVectorDecorator getLabel) . parse) <$> sourceBlobs
-  pure $! render sourceBlobs (stripDiff (diffTerms' terms))
-
+  traceEventIO $ "diffFiles@SEMANTIC-DIFF END parse terms: " <> paths
+  traceEventIO $ "diffFiles@SEMANTIC-DIFF START diff terms: " <> paths
+  traceEvent ("diffFiles@SEMANTIC-DIFF END diff terms: " <> paths) pure $!! stripDiff (diffTerms' terms)
   where
     diffTerms' terms = case runBothWith areNullOids sourceBlobs of
         (True, False) -> pure $ Insert (snd terms)
@@ -158,19 +152,18 @@ diffFiles parse render sourceBlobs = do
           runBothWith diffTerms terms
     areNullOids a b = (hasNullOid a, hasNullOid b)
     hasNullOid blob = oid blob == nullOid || null (source blob)
+    -- For trace events
+    paths = runBothWith (\a b -> fileAtSha a <> " -> " <> fileAtSha b) sourceBlobs
+    fileAtSha x = path x <> "@" <> toS (oid x)
 
-getLabel :: HasField fields Category => CofreeF (Syntax leaf) (Record fields) b -> (Category, Maybe leaf)
-getLabel (h :< t) = (category h, case t of
-  Leaf s -> Just s
-  _ -> Nothing)
+    getLabel :: HasField fields Category => CofreeF (Syntax leaf) (Record fields) b -> (Category, Maybe leaf)
+    getLabel (h :< t) = (category h, case t of
+      Leaf s -> Just s
+      _ -> Nothing)
 
--- | Determine whether two terms are comparable based on the equality of their categories.
-compareCategoryEq :: Functor f => HasField fields Category => Term f (Record fields) -> Term f (Record fields) -> Bool
-compareCategoryEq = (==) `on` category . extract
-
--- | Returns a rendered diff given a parser, diff arguments and two source blobs.
-textDiff :: (ToJSON (Record fields), DefaultFields fields) => Parser (Syntax Text) (Record fields) -> Arguments -> Both SourceBlob -> IO Output
-textDiff parser arguments = diffFiles parser $ case format arguments of
+-- | Returns a rendered diff given arguments and two source blobs.
+renderDiff :: (ToJSON (Record fields), NFData (Record fields), DefaultFields fields) => Arguments -> Both SourceBlob -> SyntaxDiff Text fields -> Output
+renderDiff args = case format args of
   Split -> split
   Patch -> patch
   SExpression -> sExpression TreeOnly
@@ -178,24 +171,8 @@ textDiff parser arguments = diffFiles parser $ case format arguments of
   Summary -> summary
   TOC -> toc
 
--- | Returns a truncated diff given diff arguments and two source blobs.
-truncatedDiff :: Arguments -> Both SourceBlob -> IO Output
-truncatedDiff Arguments{..} sources = pure $ case format of
-  Split -> SplitOutput mempty
-  Patch -> PatchOutput (truncatePatch sources)
-  SExpression -> SExpressionOutput mempty
-  JSON -> JSONOutput mempty
-  Summary -> SummaryOutput mempty
-  TOC -> TOCOutput mempty
-
--- | Prints a rendered diff to stdio or a filepath given a parser, diff arguments and two source blobs.
-printDiff :: (ToJSON (Record fields), DefaultFields fields) => Parser (Syntax Text) (Record fields) -> Arguments -> Both SourceBlob -> IO ByteString
-printDiff parser arguments sources = do
-  rendered <- textDiff parser arguments sources
-  pure $ case rendered of
-    SplitOutput text -> encodeUtf8 text
-    PatchOutput text -> encodeUtf8 text
-    SExpressionOutput text -> text
-    JSONOutput series -> toS $ encode series
-    SummaryOutput summaries -> toS $ encode summaries
-    TOCOutput summaries -> toS $ encode summaries
+-- | Prints a rendered diff to stdio or a filepath given a parser, arguments and two source blobs.
+printDiff :: (ToJSON (Record fields), NFData (Record fields), DefaultFields fields) => Parser (Syntax Text) (Record fields) -> Arguments -> Both SourceBlob -> IO ByteString
+printDiff parser args sources = do
+  diff <- diffFiles parser sources
+  pure $! concatOutputs [renderDiff args sources diff]
diff --git a/src/Patch.hs b/src/Patch.hs
index 0e976a91c..b574ad3ec 100644
--- a/src/Patch.hs
+++ b/src/Patch.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE DeriveAnyClass #-}
 {-# OPTIONS_GHC -funbox-strict-fields #-}
 module Patch
 ( Patch(..)
@@ -24,7 +25,7 @@ data Patch a
   = Replace a a
   | Insert a
   | Delete a
-  deriving (Eq, Foldable, Functor, Generic, Ord, Show, Traversable)
+  deriving (Eq, Foldable, Functor, Generic, Ord, Show, Traversable, NFData)
 
 
 -- DSL
diff --git a/src/Range.hs b/src/Range.hs
index 642292271..bc4a8d51e 100644
--- a/src/Range.hs
+++ b/src/Range.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE DeriveAnyClass #-}
 module Range where
 
 import qualified Data.Char as Char
@@ -10,7 +11,7 @@ import Test.LeanCheck
 
 -- | A half-open interval of integers, defined by start & end indices.
 data Range = Range { start :: Int, end :: Int }
-  deriving (Eq, Show, Generic)
+  deriving (Eq, Show, Generic, NFData)
 
 -- | Make a range at a given index.
 rangeAt :: Int -> Range
diff --git a/src/SourceSpan.hs b/src/SourceSpan.hs
index 12dfe3594..33d5c8cf5 100644
--- a/src/SourceSpan.hs
+++ b/src/SourceSpan.hs
@@ -26,7 +26,7 @@ data SourcePos = SourcePos
     -- Column number
     --
   , column :: Int
-  } deriving (Show, Read, Eq, Ord, Generic, Hashable)
+  } deriving (Show, Read, Eq, Ord, Generic, Hashable, NFData)
 
 displaySourcePos :: SourcePos -> Text
 displaySourcePos SourcePos{..} =
@@ -49,7 +49,7 @@ data SourceSpan = SourceSpan
     -- End of the span
     --
   , spanEnd :: SourcePos
-  } deriving (Show, Read, Eq, Ord, Generic, Hashable)
+  } deriving (Show, Read, Eq, Ord, Generic, Hashable, NFData)
 
 displayStartEndPos :: SourceSpan -> Text
 displayStartEndPos sp =
diff --git a/src/Syntax.hs b/src/Syntax.hs
index 6913aa166..64dc4b0f8 100644
--- a/src/Syntax.hs
+++ b/src/Syntax.hs
@@ -106,7 +106,7 @@ data Syntax a f
   | Ty [f]
   -- | A send statement has a channel and an expression in Go.
   | Send f f
-  deriving (Eq, Foldable, Functor, Generic, Generic1, Mergeable, Ord, Show, Traversable, ToJSON)
+  deriving (Eq, Foldable, Functor, Generic, Generic1, Mergeable, Ord, Show, Traversable, ToJSON, NFData)
 
 
 -- Instances
diff --git a/src/Term.hs b/src/Term.hs
index 6910f005f..0ed7e35e8 100644
--- a/src/Term.hs
+++ b/src/Term.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE RankNTypes, TypeFamilies, TypeSynonymInstances #-}
+{-# LANGUAGE RankNTypes, TypeFamilies, TypeSynonymInstances, UndecidableInstances #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 module Term where
 
@@ -17,6 +17,12 @@ type TermF = CofreeF
 type SyntaxTerm leaf fields = Term (Syntax leaf) (Record fields)
 type SyntaxTermF leaf fields = TermF (Syntax leaf) (Record fields)
 
+instance (NFData (f (Cofree f a)), NFData a, Functor f) => NFData (Cofree f a) where
+  rnf = rnf . runCofree
+
+instance (NFData a, NFData (f b)) => NFData (CofreeF f a b) where
+  rnf (a :< s) = rnf a `seq` rnf s `seq` ()
+
 -- | Zip two terms by combining their annotations into a pair of annotations.
 -- | If the structure of the two terms don't match, then Nothing will be returned.
 zipTerms :: (Traversable f, GAlign f) => Term f annotation -> Term f annotation -> Maybe (Term f (Both annotation))
diff --git a/test/IntegrationSpec.hs b/test/IntegrationSpec.hs
index a2c52b957..f85087b66 100644
--- a/test/IntegrationSpec.hs
+++ b/test/IntegrationSpec.hs
@@ -115,11 +115,12 @@ testParse path expectedOutput = do
   actual `shouldBe` expected
 
 testDiff :: Renderer (Record '[Range, Category, SourceSpan]) -> Both FilePath -> FilePath -> Expectation
-testDiff renderer paths diff = do
+testDiff renderer paths expectedOutput = do
   sources <- traverse readAndTranscodeFile' paths
-  diff' <- diffFiles parser renderer (sourceBlobs sources)
-  let actual = (Verbatim . stripWhitespace. concatOutputs . pure) diff'
-  expected <- (Verbatim . stripWhitespace) <$> B.readFile diff
+  diff <- diffFiles parser (sourceBlobs sources)
+  let diffOutput = renderer (sourceBlobs sources) diff
+  let actual = (Verbatim . stripWhitespace. concatOutputs . pure) diffOutput
+  expected <- (Verbatim . stripWhitespace) <$> B.readFile expectedOutput
   actual `shouldBe` expected
   where
     parser = parserForFilepath filePath
diff --git a/test/TOCSpec.hs b/test/TOCSpec.hs
index 9a298e77c..a054d2dee 100644
--- a/test/TOCSpec.hs
+++ b/test/TOCSpec.hs
@@ -110,19 +110,23 @@ spec = parallel $ do
   describe "diffFiles" $ do
     it "encodes to final JSON" $ do
       sourceBlobs <- blobsForPaths (both "ruby/methods.A.rb" "ruby/methods.B.rb")
-      let parser = parserForFilepath (path (fst sourceBlobs))
-      output <- diffFiles parser toc sourceBlobs
-      concatOutputs (pure output) `shouldBe` ("{\"changes\":{\"ruby/methods.A.rb -> ruby/methods.B.rb\":[{\"span\":{\"start\":[1,1],\"end\":[2,4]},\"category\":\"Method\",\"term\":\"self.foo\",\"changeType\":\"added\"},{\"span\":{\"start\":[4,1],\"end\":[6,4]},\"category\":\"Method\",\"term\":\"bar\",\"changeType\":\"modified\"},{\"span\":{\"start\":[4,1],\"end\":[5,4]},\"category\":\"Method\",\"term\":\"baz\",\"changeType\":\"removed\"}]},\"errors\":{}}" )
+      output <- diffOutput sourceBlobs
+      output `shouldBe` "{\"changes\":{\"ruby/methods.A.rb -> ruby/methods.B.rb\":[{\"span\":{\"start\":[1,1],\"end\":[2,4]},\"category\":\"Method\",\"term\":\"self.foo\",\"changeType\":\"added\"},{\"span\":{\"start\":[4,1],\"end\":[6,4]},\"category\":\"Method\",\"term\":\"bar\",\"changeType\":\"modified\"},{\"span\":{\"start\":[4,1],\"end\":[5,4]},\"category\":\"Method\",\"term\":\"baz\",\"changeType\":\"removed\"}]},\"errors\":{}}"
 
     it "encodes to final JSON if there are parse errors" $ do
       sourceBlobs <- blobsForPaths (both "ruby/methods.A.rb" "ruby/methods.X.rb")
-      let parser = parserForFilepath (path (fst sourceBlobs))
-      output <- diffFiles parser toc sourceBlobs
-      concatOutputs (pure output) `shouldBe` ("{\"changes\":{},\"errors\":{\"ruby/methods.A.rb -> ruby/methods.X.rb\":[{\"span\":{\"start\":[1,1],\"end\":[3,1]},\"error\":\"def bar\\nen\\n\"}]}}" )
+      output <- diffOutput sourceBlobs
+      output `shouldBe` "{\"changes\":{},\"errors\":{\"ruby/methods.A.rb -> ruby/methods.X.rb\":[{\"span\":{\"start\":[1,1],\"end\":[3,1]},\"error\":\"def bar\\nen\\n\"}]}}"
 
 type Diff' = SyntaxDiff String '[Range, Category, SourceSpan]
 type Term' = SyntaxTerm String '[Range, Category, SourceSpan]
 
+diffOutput :: Both SourceBlob -> IO ByteString
+diffOutput sourceBlobs = do
+  let parser = parserForFilepath (path (fst sourceBlobs))
+  diff <- diffFiles parser sourceBlobs
+  pure $ concatOutputs [toc sourceBlobs diff]
+
 numTocSummaries :: Diff' -> Int
 numTocSummaries diff = Prologue.length $ filter (not . isErrorSummary) (diffTOC blankDiffBlobs diff)
 
@@ -184,17 +188,9 @@ isMethodOrFunction a = case runCofree (unListableF a) of
   _ -> False
 
 testDiff :: Both SourceBlob -> IO (Diff (Syntax Text) (Record '[Range, Category, SourceSpan]))
-testDiff sourceBlobs = do
-  terms <- traverse (fmap (defaultFeatureVectorDecorator getLabel) . parser) sourceBlobs
-  pure $! stripDiff (diffTerms' terms sourceBlobs)
+testDiff sourceBlobs = diffFiles parser sourceBlobs
   where
     parser = parserForFilepath (path . fst $ sourceBlobs)
-    diffTerms' terms blobs = case runBothWith areNullOids blobs of
-      (True, False) -> pure $ Insert (snd terms)
-      (False, True) -> pure $ Delete (fst terms)
-      (_, _) -> runBothWith diffTerms terms
-    areNullOids a b = (hasNullOid a, hasNullOid b)
-    hasNullOid blob = oid blob == nullOid || Source.null (source blob)
 
 blobsForPaths :: Both FilePath -> IO (Both SourceBlob)
 blobsForPaths paths = do