The full quote is: "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

The full quote is: "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%." It doesn't mean "don't optimize" at all. It means: optimize those parts that actually make the application slow. I can tell from experience that intuition has repeatedly failed me about which parts those are. Measure, don't guess, before you optimize.