Ask HN: How do I go about optimizing a codebase?

Say I wanted to improve the performance of a large codebase such as a compiler, runtime, web server, etc.

How do you do this? I guess you can profile it but there’s so much information that the profiler gives and I usually don’t know where to look. How do you know where to look? Are there any books, blogposts that are good to start learning how to do this?