Isn't this at least in part because google was caching the pages & serving them (possibly pre-loading them, I don't know) from their own Google CDN? So comparing the speed of loading a performant page
from google's CDN from a SERP click vs. loading the page from the origin is not really a fair comparison. My website would be faster if it were in a warm google CDN cache as well :)
https://developers.google.com/amp/cache