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.

Yep. Also:

The rust borrow checker makes it difficult to implement tree like structures with pointers like you would in C or C++. Safe rust trees are (imo) best written using vecs.

Rust makes function arguments noalias.

Rust adds runtime array bound checks.

Rust and C++ do iteration quite differently. Rust encourages map/filter/reduce. I suspect this results in different assembly.

Doing a “unity build” in rust is really easy (codegen-units=1). In C++, you need to make heavy modifications to your build system and sometimes your source too.

But with some time you could probably port the optimisations across. If anyone has some spare tokens, Claude can be quite good at doing this sort of work. Show it both repositories and tell it to make the C++ code just as fast as rust.