If he is one of the devs between both libraries, why is there a performance gap? Why not port the optimizations from one lib to the other? You can even create a pinned agent workflow that automatically translates optimizations between repos. Fairly trivial to implement actually.
Because developers of security critical code don't flippantly merge LLM changes for marginal performance gain, and their time is incredibly valuable.
Just because you can theoretically write equivalent code in both languages doesn't mean two idiomatic implementations in each language will be 1:1 with each other. I haven't looked at the code in question, but some examples of common differences:
A C++ program might do template metaprogramming at compile time and the same thing at runtime in Rust, or vice versa. The C++ version might use virtual functions that rust wouldn't use. The Rust version might simply give more optimization information to the backend. The C++ version might use fairly awful parts of the stdlib like iostreams or shared_ptr that rust simply implements better. Etc.
Or maybe they're just focused on the rust implementation as they should be.