Thanks Pancho for this one:

Performance breakthroughs seem to come in two varieties: easy and hard. That's no platitude; the boundary between the two is surprisingly clear. Although in some cases it has taken considerable genius to realize their first application, they're easy to understand. The other kind involve careful measurement, specific knowledge, and a fair amount of tuning. Good programmers can operate in either the "hard" or "easy" mode. This article offers a paired collection of 4 hard and easy tales from real (programming) life.