Earlier quoted context omitted.
If all websites fall into one of two categories: fast or slow, and you penalize one, it's the same as helping the other and vice versa.
The world isn't that simple. Search engines know how fast their fetches are all of the time, but they only know how fast a javascript-heavy site is when the search engine crawls with javascript on -- which is rarely. And by construction, AMP pages are easy to analyze without having to execute anything.
(disclaimer: I work at Google, and used to work on Search)