More specifically, they're competing to produce the fastest code for software that follows the C specification to the letter. That's not necessarily the same as producing the fastest code that actually achieves the intended goal.

For example, if I recall correctly the popular Opus codec overflows signed integers when decoding invalid data, and so long as this can be guaranteed to produce some (possibly implementation-specific) result this is perfectly safe. However, this is technically undefined behaviour - a particularly malevolent optimising C compiler could decide to give the sender of the data arbitrary code execution, because it's allowed to do whatever it likes. This might even make the code run faster, but it'd make decoding Opus correctly and safely slower because the decoder would have to do a bunch of gratuitous overflow checks on operations it could otherwise just let overflow. Fortunately, gcc hasn't reached that level of advanced malevolence yet.