Consider using Data.ByteString.Encodings.isAscii for checking ByteString asciiness

[chessai]

https://hackage.haskell.org/package/bytestring-encodings-0.2.0.2/docs/Data-ByteString-Encodings.html#v:isAscii

This function is about 8x faster than a call to Data.ByteString.all (< 128), because instead of checking one Word8 at a time, it can check eight Word8s at a time, using a cool bit twiddling optimisation. There is a test suite and benchmark suite to confirm that it both works and is that fast.