diff --git a/src/Data/ByteArray/Builder.hs b/src/Data/ByteArray/Builder.hs
index cc19f98..b42000b 100644
--- a/src/Data/ByteArray/Builder.hs
+++ b/src/Data/ByteArray/Builder.hs
@@ -101,6 +101,7 @@ module Data.ByteArray.Builder
   , flush
   ) where
 
+import Control.Exception (SomeException,toException)
 import Control.Monad.ST (ST,runST)
 import Control.Monad.IO.Class (MonadIO,liftIO)
 import Data.ByteArray.Builder.Unsafe (Builder(Builder))
@@ -108,14 +109,14 @@ import Data.ByteArray.Builder.Unsafe (BuilderState(BuilderState),pasteIO)
 import Data.ByteArray.Builder.Unsafe (Commits(Initial,Mutable,Immutable))
 import Data.ByteArray.Builder.Unsafe (reverseCommitsOntoChunks)
 import Data.ByteArray.Builder.Unsafe (stringUtf8,cstring)
+import Data.ByteArray.Builder.Unsafe (addCommitsLength,copyReverseCommits)
 import Data.ByteString.Short.Internal (ShortByteString(SBS))
 import Data.Bytes.Chunks (Chunks(ChunksNil))
-import Data.Bytes.Types (Bytes(Bytes))
+import Data.Bytes.Types (Bytes(Bytes),MutableBytes(MutableBytes))
 import Data.Char (ord)
 import Data.Foldable (foldlM)
 import Data.Int (Int64,Int32,Int16,Int8)
 import Data.Primitive (ByteArray(..),MutableByteArray(..),PrimArray(..))
-import Data.Primitive.Unlifted.Array (MutableUnliftedArray,UnliftedArray)
 import Data.Text.Short (ShortText)
 import Data.WideWord (Word128)
 import Data.Word (Word64,Word32,Word16,Word8)
@@ -127,11 +128,9 @@ import GHC.ST (ST(ST))
 
 import qualified Arithmetic.Nat as Nat
 import qualified Arithmetic.Types as Arithmetic
-import qualified Control.Monad.Primitive as PM
 import qualified Data.ByteArray.Builder.Bounded as Bounded
 import qualified Data.ByteArray.Builder.Bounded.Unsafe as UnsafeBounded
 import qualified Data.Primitive as PM
-import qualified Data.Primitive.Unlifted.Array as PM
 import qualified Data.Text.Short as TS
 import qualified GHC.Exts as Exts
 
@@ -152,27 +151,49 @@ run hint@(I# hint# ) (Builder f) = runST $ do
 -- the callback escape from the callback (i.e. do not write it to an
 -- @IORef@). Also, do not @unsafeFreezeByteArray@ any of the mutable
 -- byte arrays in the callback. The intent is that the callback will
--- write the buffers out, preferably using vectored I/O.
+-- write the buffer out.
 putMany :: Foldable f
   => Int -- ^ Size of shared chunk (use 8176 if uncertain)
   -> (a -> Builder) -- ^ Value builder
   -> f a -- ^ Collection of values
-  -> (UnliftedArray (MutableByteArray RealWorld) -> IO b) -- ^ Consume chunks.
+  -> (MutableBytes RealWorld -> IO b) -- ^ Consume chunks.
   -> IO ()
 {-# inline putMany #-}
-putMany hint@(I# hint#) g xs cb = do
+putMany hint0 g xs cb = do
   MutableByteArray buf0 <- PM.newByteArray hint
   BuilderState bufZ offZ _ cmtsZ <- foldlM
     (\st0 a -> do
       st1@(BuilderState buf off _ cmts) <- pasteIO (g a) st0
       case cmts of
-        Initial -> pure st1
+        Initial -> if I# off < threshold
+          then pure st1
+          else do
+            _ <- cb (MutableBytes (MutableByteArray buf) 0 (I# off))
+            pure (BuilderState buf0 0# hint# Initial)
         _ -> do
-          _ <- cb =<< commitsToArray buf off cmts
-          pure (BuilderState buf0 0# hint# Initial)
+          let total = addCommitsLength (I# off) cmts
+              doff0 = total - I# off
+          large <- PM.newByteArray total
+          stToIO (PM.copyMutableByteArray large doff0 (MutableByteArray buf) 0 (I# off))
+          r <- stToIO (copyReverseCommits large doff0 cmts)
+          case r of
+            0 -> do
+              _ <- cb (MutableBytes large 0 total)
+              pure (BuilderState buf0 0# hint# Initial)
+            _ -> IO (\s0 -> Exts.raiseIO# putManyError s0)
     ) (BuilderState buf0 0# hint# Initial) xs
-  _ <- cb =<< commitsToArray bufZ offZ cmtsZ
+  _ <- case cmtsZ of
+    Initial -> cb (MutableBytes (MutableByteArray bufZ) 0 (I# offZ))
+    _ -> IO (\s0 -> Exts.raiseIO# putManyError s0)
   pure ()
+  where
+  !hint@(I# hint#) = max hint0 8
+  !threshold = div (hint * 3) 4
+
+putManyError :: SomeException
+{-# noinline putManyError #-}
+putManyError = toException
+  (userError "small-bytearray-builder: putMany implementation error")
 
 -- | Variant of 'putMany' that prefixes each pushed array of chunks
 -- with the number of bytes that the chunks in each batch required.
@@ -184,75 +205,54 @@ putManyConsLength :: (Foldable f, MonadIO m)
   -> Int -- ^ Size of shared chunk (use 8176 if uncertain)
   -> (a -> Builder) -- ^ Value builder
   -> f a -- ^ Collection of values
-  -> (UnliftedArray (MutableByteArray RealWorld) -> m b) -- ^ Consume chunks.
+  -> (MutableBytes RealWorld -> m b) -- ^ Consume chunks.
   -> m ()
 {-# inline putManyConsLength #-}
 putManyConsLength n buildSize hint g xs cb = do
   let !(I# n# ) = Nat.demote n
   let !(I# actual# ) = max hint (I# n# )
+  let !threshold = div (I# actual# * 3) 4
   MutableByteArray buf0 <- liftIO (PM.newByteArray (I# actual# ))
   BuilderState bufZ offZ _ cmtsZ <- foldlM
     (\st0 a -> do
       st1@(BuilderState buf off _ cmts) <- liftIO (pasteIO (g a) st0)
       case cmts of
-        Initial -> pure st1
+        Initial -> if I# off < threshold
+          then pure st1
+          else do
+            let !dist = off -# n#
+            _ <- liftIO $ stToIO $ UnsafeBounded.pasteST
+              (buildSize (fromIntegral (I# dist)))
+              (MutableByteArray buf0) 0
+            _ <- cb (MutableBytes (MutableByteArray buf) 0 (I# off))
+            pure (BuilderState buf0 n# (actual# -# n# ) Initial)
         _ -> do
           let !dist = commitDistance1 buf0 n# buf off cmts
           _ <- liftIO $ stToIO $ UnsafeBounded.pasteST
             (buildSize (fromIntegral (I# dist)))
-            (MutableByteArray buf0)
-            0
-          _ <- cb =<< liftIO (IO (PM.internal (commitsToArray buf off cmts)))
-          pure (BuilderState buf0 n# (actual# -# n# ) Initial)
+            (MutableByteArray buf0) 0
+          let total = addCommitsLength (I# off) cmts
+              doff0 = total - I# off
+          large <- liftIO (PM.newByteArray total)
+          liftIO (stToIO (PM.copyMutableByteArray large doff0 (MutableByteArray buf) 0 (I# off)))
+          r <- liftIO (stToIO (copyReverseCommits large doff0 cmts))
+          case r of
+            0 -> do
+              _ <- cb (MutableBytes large 0 total)
+              pure (BuilderState buf0 n# (actual# -# n# ) Initial)
+            _ -> liftIO (IO (\s0 -> Exts.raiseIO# putManyError s0))
     ) (BuilderState buf0 n# (actual# -# n# ) Initial) xs
-  let !distZ = commitDistance1 bufZ n# bufZ offZ cmtsZ
-  _ <- liftIO $ stToIO $ UnsafeBounded.pasteST
-    (buildSize (fromIntegral (I# distZ)))
-    (MutableByteArray buf0)
-    0
-  _ <- cb =<< liftIO (IO (PM.internal (commitsToArray bufZ offZ cmtsZ)))
+  _ <- case cmtsZ of
+    Initial -> do
+      let !distZ = offZ -# n#
+      _ <- liftIO $ stToIO $ UnsafeBounded.pasteST
+        (buildSize (fromIntegral (I# distZ)))
+        (MutableByteArray buf0)
+        0
+      cb (MutableBytes (MutableByteArray bufZ) 0 (I# offZ))
+    _ -> liftIO (IO (\s0 -> Exts.raiseIO# putManyError s0))
   pure ()
 
-commitsToArray ::
-     MutableByteArray# RealWorld -- final chunk to append to commits
-  -> Int# -- offset
-  -> Commits RealWorld
-  -> IO (UnliftedArray (MutableByteArray RealWorld))
-commitsToArray buf off cmts = do
-  let ct = countCommits 1 cmts
-  bufs <- PM.unsafeNewUnliftedArray ct
-  -- Only shrink the last chunk. Crucially, this is never the first
-  -- chunk (except on the commitsToArray call at the end of folding
-  -- over the collection). We only perform this shrink in the hopes
-  -- that a future GHC will allow reclaiming bytes from shrunk arrays.
-  shrinkMutableByteArray (MutableByteArray buf) (I# off)
-  PM.writeUnliftedArray bufs (ct - 1) (MutableByteArray buf)
-  writeCommitsToArray (ct - 2) bufs cmts
-  PM.unsafeFreezeUnliftedArray bufs
-
--- See the documentation for putMany.
-writeCommitsToArray ::
-     Int
-  -> MutableUnliftedArray RealWorld (MutableByteArray RealWorld)
-  -> Commits RealWorld
-  -> IO ()
-writeCommitsToArray !ix !arrs x0 = case x0 of
-  Initial -> pure ()
-  Mutable buf _ x1 -> do
-    PM.writeUnliftedArray arrs ix (MutableByteArray buf)
-    writeCommitsToArray (ix - 1) arrs x1
-  Immutable arr off len x1 -> do
-    buf <- PM.newByteArray (I# len)
-    PM.copyByteArray buf 0 (ByteArray arr) (I# off) (I# len)
-    PM.writeUnliftedArray arrs ix buf
-    writeCommitsToArray (ix - 1) arrs x1
-
-countCommits :: Int -> Commits s -> Int
-countCommits !n x0 = case x0 of
-  Initial -> n
-  Mutable _ _ x1 -> countCommits (n + 1) x1
-  Immutable _ _ _ x1 -> countCommits (n + 1) x1
-
 -- | Convert a bounded builder to an unbounded one. If the size
 -- is a constant, use @Arithmetic.Nat.constant@ as the first argument
 -- to let GHC conjure up this value for you.
@@ -904,7 +904,3 @@ indexChar8Array (ByteArray b) (I# i) = C# (Exts.indexCharArray# b i)
 
 c2w :: Char -> Word8
 c2w = fromIntegral . ord
-
-shrinkMutableByteArray :: MutableByteArray RealWorld -> Int -> IO ()
-shrinkMutableByteArray (MutableByteArray x) (I# i) =
-  IO (\s -> (# Exts.shrinkMutableByteArray# x i s, ()#))
diff --git a/src/Data/ByteArray/Builder/Unsafe.hs b/src/Data/ByteArray/Builder/Unsafe.hs
index 2cc3716..54a480e 100644
--- a/src/Data/ByteArray/Builder/Unsafe.hs
+++ b/src/Data/ByteArray/Builder/Unsafe.hs
@@ -18,6 +18,8 @@ module Data.ByteArray.Builder.Unsafe
   , fromEffect
     -- * Finalization
   , reverseCommitsOntoChunks
+  , copyReverseCommits
+  , addCommitsLength
     -- * Safe Functions
     -- | These functions are actually completely safe, but they are defined
     -- here because they are used by typeclass instances. Import them from
@@ -102,6 +104,13 @@ data Commits s
       !(Commits s)
   | Initial
 
+-- | Add the total number of bytes in the commits to first
+-- argument.
+addCommitsLength :: Int -> Commits s -> Int
+addCommitsLength !acc Initial = acc
+addCommitsLength !acc (Immutable _ _ x cs) = addCommitsLength (acc + I# x) cs
+addCommitsLength !acc (Mutable _ x cs) = addCommitsLength (acc + I# x) cs
+
 -- | Cons the chunks from a list of @Commits@ onto an initial
 -- @Chunks@ list (this argument is often @ChunksNil@). This reverses
 -- the order of the chunks, which is desirable since builders assemble
@@ -120,6 +129,37 @@ reverseCommitsOntoChunks !xs (Mutable buf len cs) = case len of
     arr <- PM.unsafeFreezeByteArray (MutableByteArray buf)
     reverseCommitsOntoChunks (ChunksCons (Bytes arr 0 (I# len)) xs) cs
 
+-- | Copy the contents of the chunks into a mutable array, reversing
+-- the order of the chunks.
+-- Precondition: The destination must have enough space to house the
+-- contents. This is not checked.
+copyReverseCommits ::
+     MutableByteArray s -- ^ Destination
+  -> Int -- ^ Destination range successor
+  -> Commits s -- ^ Source
+  -> ST s Int
+{-# inline copyReverseCommits #-}
+copyReverseCommits (MutableByteArray dst) (I# off) cs = ST
+  (\s0 -> case copyReverseCommits# dst off cs s0 of
+    (# s1, nextOff #) -> (# s1, I# nextOff #)
+  )
+
+copyReverseCommits# ::
+     MutableByteArray# s
+  -> Int#
+  -> Commits s
+  -> State# s
+  -> (# State# s, Int# #)
+copyReverseCommits# _ off Initial s0 = (# s0, off #)
+copyReverseCommits# marr prevOff (Mutable arr sz cs) s0 =
+  let !off = prevOff -# sz in
+  case Exts.copyMutableByteArray# arr 0# marr off sz s0 of
+    s1 -> copyReverseCommits# marr off cs s1
+copyReverseCommits# marr prevOff (Immutable arr soff sz cs) s0 =
+  let !off = prevOff -# sz in
+  case Exts.copyByteArray# arr soff marr off sz s0 of
+    s1 -> copyReverseCommits# marr off cs s1
+
 -- | Create a builder from a cons-list of 'Char'. These
 -- are be UTF-8 encoded.
 stringUtf8 :: String -> Builder
diff --git a/test/Main.hs b/test/Main.hs
index a533c65..ddd738e 100644
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -8,7 +8,7 @@
 import Control.Applicative (liftA2)
 import Control.Monad.ST (runST)
 import Data.ByteArray.Builder
-import Data.Bytes.Types (Bytes(Bytes))
+import Data.Bytes.Types (Bytes(Bytes),MutableBytes(MutableBytes))
 import Data.Bytes.Chunks (Chunks(ChunksNil,ChunksCons))
 import Data.Primitive (PrimArray)
 import Data.Word
@@ -30,7 +30,6 @@ import qualified Data.ByteString.Lazy.Char8 as LB
 import qualified Data.Bytes.Chunks as Chunks
 import qualified Data.List as L
 import qualified Data.Primitive as PM
-import qualified Data.Primitive.Unlifted.Array as PM
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as TE
 import qualified GHC.Exts as Exts
@@ -204,15 +203,13 @@ tests = testGroup "Tests"
     [ THU.testCase "A" $ do
         ref <- newIORef []
         let txt = "hello_world_are_you_listening" :: [Char]
-        putMany 7 ascii txt (ontoRef ref)
+        putMany 7 ascii txt (bytesOntoRef ref)
         res <- readIORef ref
         id $
-          [ map c2w "hello_w"
-          , map c2w "o"
-          , map c2w "rld_are"
-          , map c2w "_"
-          , map c2w "you_lis"
-          , map c2w "t"
+          [ map c2w "hello_"
+          , map c2w "world_"
+          , map c2w "are_yo"
+          , map c2w "u_list"
           , map c2w "ening"
           ] @=? map Exts.toList (Exts.toList res)
     ]
@@ -222,33 +219,26 @@ tests = testGroup "Tests"
         let txt = "hello_world_are_you_listening" :: [Char]
         putManyConsLength Nat.constant
           (\n -> Bounded.word16BE (fromIntegral n))
-          13 ascii txt (ontoRef ref)
+          16 ascii txt (bytesOntoRef ref)
         res <- readIORef ref
         id $
-          [ 0x00 : 0x0C : map c2w "hello_world"
-          , map c2w "_"
-          , 0x00 : 0x0C : map c2w "are_you_lis"
-          , map c2w "t"
-          , 0x00 : 0x05 : map c2w "ening"
+          [ 0x00 : 0x0A : map c2w "hello_worl"
+          , 0x00 : 0x0A : map c2w "d_are_you_"
+          , 0x00 : 0x09 : map c2w "listening"
           ] @=? map Exts.toList (Exts.toList res)
     ]
   ]
 
-ontoRef ::
+bytesOntoRef ::
      IORef [PM.ByteArray]
-  -> PM.UnliftedArray (PM.MutableByteArray Exts.RealWorld)
+  -> MutableBytes Exts.RealWorld
   -> IO ()
-ontoRef !ref xs = do
+bytesOntoRef !ref (MutableBytes buf off len) = do
   rs <- readIORef ref
-  ps <- PM.foldlUnliftedArrayM'
-    (\ys buf -> do
-      len <- PM.getSizeofMutableByteArray buf
-      dst <- PM.newByteArray len
-      PM.copyMutableByteArray dst 0 buf 0 len
-      dst' <- PM.unsafeFreezeByteArray dst
-      pure (ys ++ [dst'])
-    ) [] xs
-  writeIORef ref (rs ++ ps)
+  dst <- PM.newByteArray len
+  PM.copyMutableByteArray dst 0 buf off len
+  dst' <- PM.unsafeFreezeByteArray dst
+  writeIORef ref (rs ++ [dst'])
 
 instance Arbitrary Chunks where
   arbitrary = do