This is a common misconception. In key benchmarks, it is the same speed (a few still lag, but not due to inherent weaknesses in the language). Earlier versions of rust were slower because the compiler didn't have the insane optimizations that many C++ compilers have.
In fact, it may be easier for developers to write code that runs faster with rust than C++, thanks to much error and exception handling code that you simply do not need in Rust.
Most importantly, however, is finding Rust developers who are experienced enough to write high-performance Rust. I would imagine that it's a lot easier to find C++ devs.