An oft-heard adage in business and life. It serves as the moral to one of Aesop's best known fables, The Tortoise and the Hare. I think a more precise moral to the story would have been "Slow but steady wins the race against fast but stupid."
Honestly, this always struck me as more a polite kind of humouring than genuinely wise advice. After all, this isn't a heroic, John Henry-style tale here. A cynic would be quick to point out that the tortoise won the race only because he was lucky enough to have a lazy and distractible slob for an opponent. Had our slow-but-steady turtle been up against something that was fast and steady -- say, a gazelle-- I think it's fair to say that the guy would be pretty much screwed.
Fast and steady strikes me as a much more potent combination. About the only way you're going to beat fast and steady is if you happen to be very fast and predatory.
Or slow but well-armed.
Now there's a fable I wouldn't mind reading...
(Note: This is why RoguePoet is not allowed to write children's books.)