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GNU coreutils' wc (which is the reference) does not seem to be using threads, no.

At least not at all obviously. No obvious header included and no mention of threads. I only checked the code [1] very quickly, though.

[1] https://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;...

I saw that. If adding threads makes a program faster, that implies that more CPU can make it go faster. If the filesystem is already streaming bytes to the program as fast as it can, adding threads just means that you have more threads waiting for bytes.

If adding threads increases the speed, this implies that the program is both CPU-bound and that it has opportunities for parallelism.

So, that sounds like it's CPU bound then...