Measurement is the most crucial aspect of building high-performance systems. You can't change what you don't measure because you won't know what effect your change has had, if any. Without measuring your application, you won't know whether it's performing well.
If you only go by when your software feels slow, then you have left it too late. You are reactively fixing a problem rather than proactively avoiding one. You must measure to achieve good performance, even though it's the feel that matters to a user.
Some books leave measurement, analysis, and profiling until the end. Yet, this is the first thing that should be considered. It's common to fix the wrong problem and optimize areas that do not have performance ...
