I used the "reference" source code linked from the original Haskell post, a BSD version hosted by Apple: https://opensource.apple.com/source/text_cmds/text_cmds-68/w...
It uses raw read() from a file descriptor and works with pipes as well. I think the only special handling for stdin vs. an actual file it has is calling fstat() if only the number of characters is requested, which shouldn't apply here.
So yes, this version does need to do more complicated I/O than a simple mmap(). And (broken record, but I'll stop after this) it's 2x as fast as my system's GNU wc (when compiled with -O3 vs. however the system wc was compiled).
> I had actually hoped Futhark would be slower sequentially
It might still turn out to be, if you see if you can get a faster C version of wc.