I prefer full quote from Knuth:

    "Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered. We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%."

I have the problems with many happy-coders that they only remember the part about about not optimizing early and often forget that part when you should measure performance, find bottlenecks and get rid of them.