Until Rust has equal meta-programming support to C++ it's always going to be "slower". That's why people always say this because it's always true there is nothing Rust can do C++ can't but there is quite a few things you can do in C++ but not in Rust.

Realistically the difference doesn't matter much and if you're writing code that must be as fast as possible your writing unsafe Rust that looks a lot more like C/C++ then anything Rust.