In addition to what CyberDildonics already said, C++ is also an (almost) superset of C. You can also inline assembly. If you find particular hot loops that need to be optimized at any point while profiling, it’s trivial to drop down levels of the stack to get the extreme performance you may need. There is no FFI needed in C++ to drop down a level. There is no barrier at all. You can write your code directly for the CPU, inline it into your code, and fine tune it as much as you want. I don’t know how that wouldn’t qualify as extreme performance.