As a general rule, 90% of the execution time of your program will be spent in 10% of its code. Profilers are tools that help you identify the 10% of hot spots that constrain the speed of your program. This is a good thing for making it faster.

But in the Unix tradition, profilers have a far more important function. They enable you not to optimize the other 90%! This is good, and not just because it saves you work. The really valuable effect is that not optimizing that 90% holds down global complexity and reduces bugs.

-- Eric Raymond, The Art of Unix Programming

(Applies to optimization in general)