Given that I'm currently spending lots of time on code which is bringing a clients server to its knees, because the original programmers also seemed to feel "performance doesn't matter", I'd have to say no, it isn't overrated. Performance is one valid metric of code quality. Whether it is the most important metric for your code (or a particular area of your code) depends entirely on what you're doing and what your specs are. Perhaps a better rule would be "Performance doesn't matter, except when it does." :-)