Faster alternatives to hFileSize to retrieve the size of a file in Haskell?

I don't know if there is a better way. RWH supplies its own wrapper to hFileSize:

getFileSize path = handle (\_ -> return Nothing) $
  bracket (openFile path ReadMode) hClose $ \h -> do
    size <- hFileSize h
    return (Just size)

It also notes that the unix-compat is available, which "provides portable implementations of parts of the unix package."

What you want are indeed getFileStatus and fileSize, both from System.Posix (which will work just fine under Windows, if you use the unix-compat package instead of unix). Usage is as follows, leaving error handling up to you:

getFileSize :: String -> IO Integer
getFileSize path = do
    stat <- getFileStatus path
    return $ fromIntegral (fileSize stat)

For what it's worth, and though I think it's less readable, you could shorten this form to:

getFileSize path = getFileStatus path >>= \s -> return $ fileSize s

It seems like System.Posix.Files doesn't work in Windows (except in Cygwin), have you tried unix-compat ?

https://hackage.haskell.org/package/unix-compat-0.4.1.4/docs/System-PosixCompat-Files.html

This worked for me on my Windows 10 machine:

> cabal install unix-compat
Resolving dependencies...
... lots of output, plus I had to put Cygwin on my path to make it build ...
> ghci
Prelude> import System.PosixCompat.Files
Prelude System.PosixCompat.Files> getFileStatus ".bashrc">>= \s -> return $ fileSize s
5764