C++ is horrible for performance. You get performance from it solely because everything is implemented pretty fast to begin with. But the second any modularity or layer of abstraction is added you immediately lose all those gains.
Other languages can handle this (MLton) gracefully, but C++ can't. The sad fact is that you often will write things in C or fortran and link to C++ to get better performance.
The better question is "If C++ is in this performance no man's land, not good enough when trying to be fast, and horribly slow when being idiomatic, why do people say it performs well at all?"