Optimizing clean code is easier, but you're gambling that your overloaded service will only lose credibility slowly enough to give you time to finish (e.g., Twitter found themselves on very thin ice for a while). Refactoring efficient code takes more work but doesn't come with that kind of deadline, it just slows down maintenance in the meantime.
Yeah, it's unfortunate that when your clean but inefficient code falls over, the engineering department takes all of the blame. But when your efficient but unclean code doesn't allow you to iterate as quickly, it's harder to place blame.
In my experience,it is much easier to find talented coders willing to improve the performance of clean (or at least sane) code than to find ones willing to endure insanity.
This should not be underestimated when facing turnover in your development team.
