The most common place where noexcept improves performance is on move constructors and move assignments when moving is cheaper than copying. If your type is not nothrow moveable std::vector will copy it instead of moving when resizing, as the move constructor throwing would leave the vector in an invalid state (while the copy constructor throwing leaves the vector unchanged).

Platforms with setjmp-longjmp based exceptions benefit greatly from noexcept as there’s setup code required before calling functions which may throw. Those platforms are now mostly gone, though. Modern “zero cost” exceptions don’t execute a single instruction related to exception handling if no exceptions are thrown (hence the name), so there just isn’t much room for noexcept to be useful to the optimizer.

Outside of those two scenarios there isn’t any reason to expect noexcept to improve performance.