An order of magnitude slower on non-trivial computation-heavy code (i.e,. not just doing a lot of IO or calls into libraries/the kernel) is a pretty good rule o

An order of magnitude slower on non-trivial computation-heavy code (i.e,. not just doing a lot of IO or calls into libraries/the kernel) is a pretty good rule of thumb. Sometimes better, sometimes much worse. The more layers of abstraction, the worse the penalty for not optimizing, so C++ is usually more heavily affected than C, for example: many of the so called "zero-cost" abstractions in C++ rely on a good optimizer.