What about this? I've tested on my pc and seems pretty fast. The trick is to generate the gen only once. Not sure if the inlines helps, though: import qualified Data.Text as T import System.Random.MWC import Control.Monad import System.IO import Data.ByteString as B import Data.Word (Word8) import Data.ByteString.Char8 as CB {- | Converts a Char to a Word8. Took from MissingH -} c2w8 :: Char -> Word8 c2w8 = fromIntegral . fromEnum charRangeStart :: Word8 charRangeStart = c2w8 'a' {-# INLINE charRangeStart #-} charRangeEnd :: Word8 charRangeEnd = c2w8 'z' {-# INLINE charRangeEnd #-} --genString :: Gen RealWorld -> IO B.ByteString genString g = do randomLen <- uniformR (50 :: Int, 450 :: Int) g str <- replicateM randomLen $ uniformR (charRangeStart, charRangeEnd) g return $ B.pack str writeCorpus :: FilePath -> IO [()] writeCorpus file = withFile file WriteMode $ \h -> do let size = 100000 _ <- withSystemRandom $ \gen -> replicateM size $ do text <- genString gen :: IO B.ByteString CB.hPutStrLn h text return [()] main :: IO [()] main = writeCorpus "test.txt" A.