Sometimes you need to profile things. This tutorial helps.
“Maybe we should stop trying the devise the ideal methods to create the ideal person and just spend time with our kids because we love them and they’ll refuse to hang out with us pretty soon.”
Source: Scratch That, Reverse It
Ian Jones recently published one of the best articles on contributing to WordPress Core that I’ve ever seen. The only two issues I saw with the post were addressed by Drew Jaynes (WP 4.2 release lead) in the first comment. I was going to write my own guide, but now I don’t have to.