Earlier quoted context omitted.
As people have pointed out, that's Chrome-only at the moment but that's also a big moving of the goal-posts. Your original comment was “far behind in a variety of features that are useful to a much bigger % of the web” — precisely how much user-visible benefit do you think comes from a feature which may accelerate the first load slightly? If that was that critical, it'd be easy to point to e.g. webpagetest.org traces…
How much user-visible benefit? Do a quick text search of this Hacker News thread for people talking about Firefox "feeling" slower than Chrome. Preload is a big underlying "why" of that experience.
I've read many comments where people mention performance in ways suggesting they have issues with UI performance, launch time, extensions, etc. Optimizing cold-cache performance the first time someone visits a site won't help with that at all.