“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 %.”
Donald Knuth, Structured Programming with go to Statements (1974) · Section on efficiency, page 268 · read it
Kept with its second sentence, which nearly everyone drops. The quote is used to excuse slow code and to forbid thinking about speed. Knuth asked for neither: measure, find the three percent, and work hard there.