Hi folks. OK, try this: darcs get http://darcs.orphi.me.uk/MyMD5 cd MyMD5 ghc -O2 --make md5sum md5sum <some large filename> On my PC, it takes 3.5 seconds to compute the MD5 hash of a 10 MB file. For comparison, the md5.exe I downloaded from the Internet does it instantaneously. It's literally so fast I can't even time it. As far as I know, my program always produces the *correct* answer, it just takes far too long to do it. If anybody has any interesting insights as to why my version is so damned slow, I'd like to hear them. (Indeed, a description of the process for tracking the problem down would also be useful!) [Much to my bemusement, when I had a look at the code, I discovered that tucked away in a corner somewhere, it reads a ByteString from disk and instantly converts it to [Word8]. Uh... oops? Anyway, eliminating this changed the numbers I get from the profiler, but it's still far too slow.]