Zlib-rs is faster than C

"faster than C" almost always boils down to different designs, implementations, algorithms, etc.

Perhaps it is faster than already-existing implementations, sure, but not "faster than C", and it is odd to make such claims.

The fact that it's faster than the C implementation that surely had more time and effort put into it doesn't look good for C here.

I think you'll find that if you re-write an application, feature-for-feature, without changing its language, the re-written version will be faster.

This is known as the Second System Effect: where Great Rewrites always succeed in making a more performant thing.