It seems like, other things equal, slow sites will do worse. So they must be getting some massive advantage to having so much cruft, otherwise they wouldn't do it.
Of my day to day, it seems like only Hacker News, Paul Graham's site, and a few other holdouts load quickly and are easy to read.
There's also less user generated content in google searches. All of that has moved to Facebook, Reddit, twitter. What remains in searches is more commercial content.
So you'll probably get cynical answers about lazy programmers and such, and there's some truth, but I'd suggest the dominant problem here is that the specification of the Web has very little consideration for possible performance built into it. Browsers are basically enormous constraint satisfaction engines, and that's intrinsically a fairly slow operation. Combine that with, well, a lot of the issues that HTTP2 is trying to address like requests for lots of small little resources, flash plugins written with their own programmer-easy-but-computer-expensive shortcuts, and the whole thing is very hard. Browsers are probably more "miraculously fast" for what they are being asked to do than surprisingly slow.