It prefers instant loading pages because that's what users want. If not, Bing, Yandex, Baidu, Yahoo, or any of the other search engines that also use AMP would simply rank differently to beat Google.
The things that's being discussed though is that even if your page loads in "no time" AMP will be perfered anyways, even if your page loads fast enough that the user wont notice a difference.
There is no fast enough that a user won't notice the difference between that and instant, especially due to RTT for the request. That's why I always click on results with the lightning icon, and that's what I figure enough other users do too in order for AMP pages to rank so highly. If that's not the case, the other search engines can rank differently to beat Google, as I said.
It prefers instant loading pages because that's what users want. If not, Bing, Yandex, Baidu, Yahoo, or any of the other search engines that also use AMP would simply rank differently to beat Google.