> they can give "fast" sites a ranking boost in search
They can already do this though, right? They can even use the same criteria.
I guess the difference is they wouldn't have to measure it themselves and they could just trust the browser to enforce it. But they already have to execute the web page to get the indexed content, and the limits they're imposing here seem to be unworkable for most SPA's, since the document-level restrictions around things like images break down if you're going a long time without reloading the page. So it's only indexed content that would ever enable this in the first place.
The only benefit I can see is that it would allow them to bake the UI into the browser, which would make it more obvious to users. And that's great, except for the tiny problem that performance isn't binary and different browsers and environments will want to have different standards.
None of the restrictions here apply to service workers. Why not? Why shouldn't browsers like Opera Mini be able to apply a different list of restrictions that include service workers? And if we're OK with browsers applying extra restrictions, than what's the value of standardization?