The whole job of programming is building air castles—it's one of the most purely mental activities you can do.
Steve McConnell, Code Complete, Second Edition

It's almost impossible to identify performance bottlenecks before a program is working completely. Programmers are very bad at guessing which four percent of the code accounts for 50 percent of the execution time, and so programmers who optimize as they go will, on average, spend 96 percent of their time optimizing code that doesn't need to be optimized. That leaves little time to optimize the four percent that really counts.

Steve McConnell, Code Complete, Second Edition

Rules of Optimization:

Rule 1. Don't do it.

Rule 2 (for experts only). Don't do it yet—that is, not until you have a perfectly clear and unoptimized solution.

M. A. Jackson

Children sometimes think that if they close their eyes and can't see you, they've made you go away. Setting a switch on the compiler to turn off warnings just means you can't see the errors. It doesn't make them go away any more than closing your eyes makes an adult go away.

Steve McConnell, Code Complete, Second Edition

You'll benefit in several ways, however, by not compiling until late in the process. The main reason is that when you compile new code, an internal stopwatch starts ticking. After the first compile, you step up the pressure: "I'll get it right with just one more compile." The "Just One More Compile" syndrome leads to hasty, error-prone changes that take more time in the long run. Avoid the rush to completion by not compiling until you've convinced yourself that the routine is right.
Steve McConnell, Code Complete, Second Edition