I haven't read the book yet, but I believe the key to a mature optimization is knowing clearly what needs to be optimized. There is no way to know it if you don't measure it. But the thing is that you may be measuring the wrong thing. If you focus on a class, for instance, without the entire context, you may be trying to speed up the wrong piece of your application.
Make sure you measure the entire flow, from the user clicking something to him seeing the response. Sometimes you will find that what needs to be optimized is not even your code.