> Above all of that real-world performance comes from proper system-level design

There are situations where "proper system-level design" just doesn't cut it, and even traditional "rewrite this module in C" doesn't work, because there is no single module to optimize, but rather system is being slowed down by many little overheads all over the place. JITs help with this, but they are not always available. It really pays off to switch to the language with less overall overhead and a focus on performance if you find yourself in such a situation.

> and using a low-level language (be it C, C++ or assembly)

C++ is not a low-level language.