AMP is a racket and I warned all our clients about it when it became this thing 3 or 4 years ago.
It was just another bullshit framework to replace HTML elements with something proprietary using Javascript and basically locks you into the Google ecosystem.
Google has since changed their approach a bit but there's currently 2000+ open issues on GitHub alone, some dating back to 2018 or earlier that'll never be fixed because they stopped pushing it
The big website performance improvement is not having CSS block your loading. And the easy fix is loading CSS async through Javascript because that's faster than waiting for all the HTML to load and then finally for the CSS to load.
So you load your 'critical' CSS for the first paint in an inline style tag to prevent content flashes and load your complete CSS files asynchronous with Javascript while the user can already see the page.
AMP did a similar trick where indeed you got faster to first paint but the site didn't really load any faster. In fact by replacing <img> with <amp-img> and other nonsense like that the end result was slower loading times.
In my view Google got lucky with AMP because loading animations were a trend back then. Sites had all these custom load bars and loading overlays so they looked more like apps I suppose but that just added a few milisecs of animation time and made loading times appear longer not shorter.
Some UX guru claimed that having a loading bar made it seem like things were loading faster but that's not true at all. That's also why most boot up and loading screens got replaced with static art instead.
In video games they remain popular but more as a sign to the user that the game is still in fact loading and hasn't locked up. And also gives you some time prepare in games where you need to jump immediately into the action.
With those Nvme SSD's taking center point I suppose loading bars will start to dissapear from games too.