What websites need to not only regularly preload+prerender other webpages but also webpages outside their trustzone? And when does the performance of half a millisecond matter?

Latency to remote servers isn’t half a millisecond. A click off from google would normally take a second or two to render and be much worse at the p95 page load time.

With AMP, this is cut down to tens of milliseconds.

> A click off from google would normally take a second or two to render

The question isn't what do people normally do. The question is what's possible without Google's help. 200ms click-to-render is not difficult.

Google can get that down to 20ms or whatever with AMP, but for 99.999% of sites, Google's monopoly position is not the thing holding you back from faster loads.

Not in my experience; AMP has no latency benefit. Additionally, this doesn't really answer the question other than "Google needs this". What other websites need to do AMP on their end?