I am not enough of a Rust compiler expert to tell you exactly where the slowdowns show up, but the common theme has been that a benchmark of a small section of

I am not enough of a Rust compiler expert to tell you exactly where the slowdowns show up, but the common theme has been that a benchmark of a small section of the critical path shows equal performance (sometimes a little faster, sometimes a little slower), and then when you expand to a larger scope, benchmarks start to slow down in comparison to their C++ alternatives. My intuitive guesses would be that the culprit is actually either larger code size, worse code layout, being unable to optimize certain code sections to the same level (undefined behavior in C++ is actually helpful for speed, and not all of it is actually unsafe), something about how calling conventions or struct layouts differ, or another "second-order" effect.