People use the premature optimization principle in exactly the wrong way these days. Knuth's 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%." That 97%/3% split is the whole point.
People bring it up to argue for never thinking about performance, which flips the intent on its head. The real takeaway is that you need to spot that critical 3% early enough to build around it, and that means doing some optimization thinking up front, not none at all.