I can't take this seriously. The claim "you can look at SOME code and see how to optimize it" is trivially true, and saying "you can't look at ALL code and see how to optimize it" changes nothing.

Exactly, you have to find the slow code first, with profiling, and then sometimes it is obvious why that code is slow. It is unlikely you can just look at a project and find the slow code in it.

You absolutely can. I've given enough examples that are obviously true. If you can't read code and reason about its performance, that is a personal issue.