C also does lack few features which can provide better performance or provide hints to the compiler.

computed goto, non-aliasing guarantees, actual "const" - just a few on top of my mind.

Generic code is generally faster in C++ unless you manually reimplement it in C.

Rust can be faster than C in few cases because stronger aliasing guarantees, but few compiler bugs prevent it.

Doing some good-performance stuff is also more difficult in C. Strings are null terminated etc..