It's not even ranked first. Rust comes a good second to third behind C & C++ on most if not all tests listed on https://sites.google.com/view/energy-efficiency-languages/re...
In the current state, much of that work is left to LLVM which is designed more around the C/C++ memory model and only some of the ownership and aliasing information can be represented in that form.
I am not arguing that Rust is faster now, only that it can be without fundamentally changing the language.
A couple of points here. Rust is generally as fast as C and C++, but on top of that it is memory and data race safe. Said another way, there is a safe language alternative which doesn’t have the pitfalls of C/C++, and that’s a great thing!