Performance is only one aspect of overall software quality, and it's usually not the most important. Finely tuned code is only one aspect of overall performance, and it's usually not the most significant. Program architecture, detailed design, and data-structure and algorithm selection usually have more influence on a program's execution speed and size than the efficiency of its code does.
Quantitative measurement is a key to maximizing performance. It's needed to find the areas in which performance improvements will really count, and it's needed again to verify that optimizations improve rather than degrade the software.
Most programs spend most of their time in a small fraction of their code. You won't know which code that is until you measure it.
Multiple iterations are usually needed to achieve desired performance improvements through code tuning.
The best way to prepare for performance work during initial coding is to write clean code that's easy to understand and modify.
