Avoid   sleep() Seriously, though, my answer is:

"Don't worry about speed before it's clear that a) speed is important, and b) speed is a problem, and c) the problem is one you can get at."

I've seen too much time go down the drain in projects when somebody decides to rework some chunk of the system to be as fast as possible, when it really didn't matter. The result is usually code that's problematic (less readable, less supportable) than if they'd just been straightforward.

Performance problems are almost always algorithmic (where "algorithm" includes the structure of SQL queries).