Zlib-rs is faster than C

"Barely" or not is completely irrelevant. The fact is that it's measurably faster than the C implementation with the more common parameters. So the point that you're trying to make isn't clear tbh.

Also I'm pretty sure that the C implementation had more man hours put into it than the Rust one.

I think that would be really hard to measure. In particular, for this sort of very optimized code, we’d want to separate out the time spent designing the algorithms (which the Rust version benefits from as well). Actually I don’t think that is possible at all (how will we separate out time spent coding experiments in C, then learning from them).

Fortunately these “which language is best” SLOC measuring contests are just frivolous little things that only silly people take seriously.