I don't have links to hand but everything I've seen shows real dropoffs in users as you increase the time. Once you're looking at low numbers of seconds you're looking at significant numbers of users simply abandoning the site. Half a second extra is not insignificant, and the user experience changes a lot between things that feel instant and things that have a noticeable wait.
Yes but you don't need AMP to have a fast loading website or even one that applies the same principles as AMP when it comes to having inline CSS, loading scripts async etc. The biggest problem in all of this is usually ads and analytics anyways.
I don't have links to hand but everything I've seen shows real dropoffs in users as you increase the time. Once you're looking at low numbers of seconds you're looking at significant numbers of users simply abandoning the site. Half a second extra is not insignificant, and the user experience changes a lot between things that feel instant and things that have a noticeable wait.