diff --git a/fs-api/bench/Main.hs b/fs-api/bench/Main.hs
new file mode 100644
index 0000000..3eb716e
--- /dev/null
+++ b/fs-api/bench/Main.hs
@@ -0,0 +1,198 @@
+{-# LANGUAGE BangPatterns       #-}
+{-# LANGUAGE DeriveAnyClass     #-}
+{-# LANGUAGE DeriveGeneric      #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE MagicHash          #-}
+{-# LANGUAGE NamedFieldPuns     #-}
+{-# LANGUAGE RecordWildCards    #-}
+{-# LANGUAGE StandaloneDeriving #-}
+
+{-# OPTIONS_GHC -Wno-orphans #-}
+
+module Main (main) where
+
+import           Control.DeepSeq (NFData (..))
+import           Control.Exception (assert)
+import           Control.Monad.Primitive (PrimMonad)
+import           Criterion.Main
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Internal as BS
+import qualified Data.ByteString.Lazy as LBS
+import           Data.Int (Int64)
+import           Data.List (unfoldr)
+import           Data.Primitive.ByteArray
+import           Data.Word (Word64)
+import           Foreign (withForeignPtr)
+import qualified GHC.ForeignPtr as GHC
+import           GHC.Generics (Generic)
+import qualified GHC.IO as GHC
+import qualified GHC.Ptr as GHC
+import           GHC.Stack (HasCallStack)
+import qualified System.Directory as Dir
+import qualified System.FS.API as FS
+import qualified System.FS.API.Lazy as FS
+import           System.FS.IO (HandleIO, ioHasBufFS, ioHasFS)
+import           System.FS.IO.Internal.Handle (HandleOS (..))
+import           System.IO.Temp (createTempDirectory,
+                     getCanonicalTemporaryDirectory)
+import           System.Random (mkStdGen, uniform)
+
+main :: IO ()
+main = do
+  putStrLn "WARNING: it is recommended to run each benchmark in isolation \
+           \with short cooldown pauses in between benchmark executable \
+           \invocations. This prevents noise coming from one benchmark \
+           \from influencing another benchmark. Example incantion: \
+           \cabal run fs-api-bench -- -m glob \"System.FS.API/hGetSome\""
+  defaultMain [benchmarks]
+
+benchmarks :: Benchmark
+benchmarks = bgroup "System.FS.API" [
+      envWithCleanup (mkFileEnv (4096 * 64) "hGetSome") cleanupFileEnv $ \ ~(hfs, _, _, fsp) ->
+        bench "hGetSome" $
+          perRunEnvWithCleanup (mkHandleEnv hfs fsp 0) (cleanupHandleEnv hfs) $ \h -> do
+            FS.hGetSome hfs h (4096 * 64)
+    , envWithCleanup (mkFileEnv (4096 * 64) "hGetSome'") cleanupFileEnv $ \ ~(hfs, hbfs, _, fsp) ->
+        bench "hGetSome'" $
+          perRunEnvWithCleanup (mkHandleEnv hfs fsp 0) (cleanupHandleEnv hfs) $ \h -> do
+            hGetSome' hbfs h (4096 * 64)
+    , envWithCleanup (mkFileEnv (4096 * 64) "hGetSomeAt") cleanupFileEnv $ \ ~(hfs, _, _, fsp) ->
+        bench "hGetSomeAt" $
+          perRunEnvWithCleanup (mkHandleEnv hfs fsp 0) (cleanupHandleEnv hfs) $ \h -> do
+            FS.hGetSomeAt hfs h (4096 * 64) 0
+    , envWithCleanup (mkFileEnv (4096 * 64) "hGetSomeAt'") cleanupFileEnv $ \ ~(hfs, hbfs, _, fsp) ->
+        bench "hGetSomeAt'" $
+          perRunEnvWithCleanup (mkHandleEnv hfs fsp 0) (cleanupHandleEnv hfs) $ \h -> do
+            hGetSomeAt' hbfs h (4096 * 64) 0
+    ]
+
+{-------------------------------------------------------------------------------
+  Benchmarkable functions
+-------------------------------------------------------------------------------}
+
+hGetSome' ::
+     (HasCallStack, PrimMonad m)
+  => FS.HasBufFS m h
+  -> FS.Handle h
+  -> Word64
+  -> m BS.ByteString
+hGetSome' hbfs !h !c = do
+    !buf <- newPinnedByteArray (fromIntegral c)
+    !c' <- FS.hGetBufSome hbfs h buf 0 (fromIntegral c)
+    ba <- unsafeFreezeByteArray buf
+    -- pure $ copyByteArrayToByteString ba 0 (fromIntegral c')
+    pure $! unsafeByteArrayToByteString ba (fromIntegral c')
+
+hGetSomeAt' ::
+     (HasCallStack, PrimMonad m)
+  => FS.HasBufFS m h
+  -> FS.Handle h
+  -> Word64
+  -> FS.AbsOffset
+  -> m BS.ByteString
+hGetSomeAt' hbfs !h !c !off = do
+    !buf <- newPinnedByteArray (fromIntegral c)
+    !c' <- FS.hGetBufSomeAt hbfs h buf 0 (fromIntegral c) off
+    ba <- unsafeFreezeByteArray buf
+    -- pure $ copyByteArrayToByteString ba 0 (fromIntegral c')
+    pure $! unsafeByteArrayToByteString ba (fromIntegral c')
+
+{-# INLINE unsafeByteArrayToByteString #-}
+unsafeByteArrayToByteString :: ByteArray -> Int -> BS.ByteString
+unsafeByteArrayToByteString !ba !len =
+    GHC.unsafeDupablePerformIO $ do
+      let !(GHC.Ptr addr#) = byteArrayContents ba
+      (MutableByteArray mba#) <- unsafeThawByteArray ba
+      let fp = GHC.ForeignPtr addr# (GHC.PlainPtr mba#)
+      BS.mkDeferredByteString fp len
+
+-- | Copy a 'Prim.ByteArray' at a certain offset and length into a
+-- 'BS.ByteString'.
+--
+-- This is a copy of a function from @cborg@.
+_copyByteArrayToByteString ::
+     ByteArray -- ^ 'ByteArray' to copy from.
+  -> Int -- ^ Offset into the 'ByteArray' to start with.
+  -> Int -- ^ Length of the data to copy.
+  -> BS.ByteString
+_copyByteArrayToByteString ba off len =
+    GHC.unsafeDupablePerformIO $ do
+      fp <- BS.mallocByteString len
+      withForeignPtr fp $ \ptr -> do
+        copyByteArrayToPtr ptr ba off len
+        return (BS.PS fp 0 len)
+
+{-------------------------------------------------------------------------------
+  Orphan instances
+-------------------------------------------------------------------------------}
+
+deriving stock instance Generic (HandleOS h)
+deriving anyclass instance NFData (HandleOS h)
+deriving anyclass instance NFData FS.FsPath
+deriving anyclass instance NFData h => NFData (FS.Handle h)
+instance NFData (FS.HasFS m h) where
+  rnf hfs =
+      dumpState `seq` hOpen `seq` hClose `seq` hIsOpen `seq` hSeek `seq`
+      hGetSome `seq`hGetSomeAt `seq` hPutSome `seq` hTruncate `seq`
+      hGetSize `seq` createDirectory `seq` createDirectoryIfMissing `seq`
+      listDirectory `seq` doesDirectoryExist `seq` doesFileExist `seq`
+      removeDirectoryRecursive `seq` removeFile `seq` renameFile `seq`
+      mkFsErrorPath `seq` unsafeToFilePath `seq` ()
+    where
+      FS.HasFS {..} = hfs
+      _coveredAllCases x = case x of
+        FS.HasFS _a _b _c _d _e _f _g _h _i _j _k _l _m _n _o _p _q _r _s _t -> ()
+
+
+instance NFData (FS.HasBufFS m h) where
+  rnf hbfs = hPutBufSome `seq` hPutBufSomeAt `seq` ()
+    where
+      FS.HasBufFS { FS.hPutBufSome , FS.hPutBufSomeAt } = hbfs
+
+{-------------------------------------------------------------------------------
+  Environment initialisation and cleanup
+-------------------------------------------------------------------------------}
+
+mkFileEnv ::
+     Int
+  -> String
+  -> IO (FS.HasFS IO HandleIO, FS.HasBufFS IO HandleIO, FilePath, FS.FsPath)
+mkFileEnv nbytes dirName = do
+    sysTmpDir <- getCanonicalTemporaryDirectory
+    tmpDir <- createTempDirectory sysTmpDir dirName
+    let hfs = ioHasFS (FS.MountPoint tmpDir)
+        hbfs = ioHasBufFS (FS.MountPoint tmpDir)
+
+    -- Create a file containing random bytes.
+    let g = mkStdGen 17
+        bytes = take nbytes $ unfoldr (Just . uniform) g
+        bs = LBS.pack bytes
+        fp = "benchfile"
+        fsp = FS.mkFsPath [fp]
+    FS.withFile hfs fsp (FS.WriteMode FS.MustBeNew) $ \h -> do
+      nbytes' <- FS.hPutAll hfs h bs
+      assert (nbytes == fromIntegral nbytes') $ pure ()
+
+    -- Read the full file into memory to make doubly sure that the file is in
+    -- the page cache, even though it might still be in the page cache as a
+    -- result of writing the file.
+    --
+    -- Having the full file in the page cache will hopefully prevent some noise
+    -- in the benchmark measurements.
+    FS.withFile hfs fsp FS.ReadMode $ \h -> do
+      bs' <- FS.hGetAll hfs h
+      pure $! rnf bs'
+
+    pure (hfs, hbfs, tmpDir, fsp)
+
+cleanupFileEnv :: (a, b, FilePath, d) -> IO ()
+cleanupFileEnv (_, _, fp, _) = Dir.removeDirectoryRecursive fp
+
+mkHandleEnv :: FS.HasFS IO HandleIO -> FS.FsPath -> Int64 -> IO (FS.Handle HandleIO)
+mkHandleEnv hfs fsp n = do
+  h <- FS.hOpen hfs fsp FS.ReadMode
+  FS.hSeek hfs h FS.AbsoluteSeek n
+  pure h
+
+cleanupHandleEnv :: FS.HasFS IO HandleIO -> FS.Handle HandleIO -> IO ()
+cleanupHandleEnv = FS.hClose
diff --git a/fs-api/fs-api.cabal b/fs-api/fs-api.cabal
index b5466cf..d2a2c29 100644
--- a/fs-api/fs-api.cabal
+++ b/fs-api/fs-api.cabal
@@ -24,7 +24,14 @@ source-repository head
   location: https://github.com/input-output-hk/fs-sim
   subdir:   fs-api
 
+common warnings
+  ghc-options:
+    -Wall -Wcompat -Wincomplete-uni-patterns
+    -Wincomplete-record-updates -Wpartial-fields -Widentities
+    -Wredundant-constraints -Wmissing-export-lists -Wunused-packages
+
 library
+  import:           warnings
   hs-source-dirs:   src
   exposed-modules:
     System.FS.API
@@ -69,12 +76,8 @@ library
     else
       hs-source-dirs: src-macos
 
-  ghc-options:
-    -Wall -Wcompat -Wincomplete-uni-patterns
-    -Wincomplete-record-updates -Wpartial-fields -Widentities
-    -Wredundant-constraints -Wmissing-export-lists -Wunused-packages
-
 test-suite fs-api-test
+  import:           warnings
   type:             exitcode-stdio-1.0
   hs-source-dirs:   test
   main-is:          Main.hs
@@ -89,8 +92,23 @@ test-suite fs-api-test
     , tasty-quickcheck
     , temporary
 
-  ghc-options:
-    -Wall -Wcompat -Wincomplete-uni-patterns
-    -Wincomplete-record-updates -Wpartial-fields -Widentities
-    -Wredundant-constraints -Wmissing-export-lists -Wunused-packages
-    -fno-ignore-asserts
+  ghc-options:      -fno-ignore-asserts
+
+benchmark fs-api-bench
+  import:           warnings
+  type:             exitcode-stdio-1.0
+  hs-source-dirs:   bench
+  main-is:          Main.hs
+  default-language: Haskell2010
+  build-depends:
+    , base
+    , bytestring
+    , criterion
+    , deepseq
+    , directory
+    , fs-api
+    , primitive
+    , random
+    , temporary
+
+  ghc-options:      -rtsopts -with-rtsopts=-T
diff --git a/fs-api/src-unix/System/FS/IO/Internal.hs b/fs-api/src-unix/System/FS/IO/Internal.hs
index b3ba47d..c26cbf4 100644
--- a/fs-api/src-unix/System/FS/IO/Internal.hs
+++ b/fs-api/src-unix/System/FS/IO/Internal.hs
@@ -1,5 +1,4 @@
-{-# LANGUAGE CPP            #-}
-{-# LANGUAGE PackageImports #-}
+{-# LANGUAGE CPP #-}
 
 -- | This is meant to be used for the implementation of HasFS instances and not
 -- directly by client code.