Donald Knuth - Premature Optimization

Donald Knuth in Computer Programming as an Art, Knuth’s Turing Award lecture (1974), printed in Communications of the ACM, Volume 17, Issue 12, Dec. 1974 p.671 (source)

Premature optimization is the root of all evil (or at least most of it) in programming.

Context (source):

“The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming.”

Also in Knuth’s paper Structured Programming with go to Statements, ACM Computing Surveys, Vol 6, No. 4, Dec. 1974 p.268:
“There is no doubt that the grail of efficiency leads to abuse. 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%. A good programmer will not be lulled into complacency by such reasoning, he will be wise to look carefully at the critical code; but only after that code has been identified.”