Salzenberg's Zeroth Rule of Optimization: "Never optimize before benchmarking."

I once wrote a communications program that used too much CPU. Turns out that its primary time sink was ... mktime()! It was converting YMDHMS into time_t for event storage. I changed the database format and got a 33% CPU time reduction.

More computing sins are committed in the name of efficiency (without necessarily achieving it) than for any other single reason -- including blind stupidity.
    -- W. A. Wulf, "A Case Against the GOTO"