Google AMP vs Mobile Ranking Question

Granted, maintaining generally good site health and hygiene is important overall, but I have always been curious about ranking in a very specific context. I look after a range of sites, some of which run AMP. For the majority, their web (desktop and mobile sites are solid (responsive, fast, etc). However, a few aren't performing as well when it comes to speed tests of their web pages but are getting good AMP traffic. How does Google look at those sites? Especially in (what I recall reading is) a mobile-first ranking world? Does the mobile/desktop experience impact overall rank (even over AMP)? Or does the relatively instant load of AMP (Google's own product) supersede any web page performance metrics? Again, very aware that a well-performing everything is ideal, but keen to ascertain the weight/priority of it all in relation to ranking. Thanks in advance!