I think the real motivation is to allow Google to use guaranteed never-slowness as an unspoofable ranking signal, essentially creating an alternative to AMP that doesn't require special tags or libraries.
Yea, that's kind of what I meant to say, although I wasn't thinking about AMP. Can Google really not use the vast amounts of data it ingests to deduce which sites have bad user experiences and are slow, though? Shouldn't that be their core competency?
Yes, Google's crawler already calculates page speed yet they've observed that few popular sites are fast. Another aspect of AMP and NSM that I didn't mention is that they are binary; this prevents sites from adding just a little more bloat and then just a a little more after that until they're quite slow.