I disagree with this. Hardware is so crazy good now relative to then, that you could put layers upon layers of abstraction (including say a 68K mac emulator compiled to javascript and running inside Firefox on x11) and the old-school gui tasks of the 90s still purr.

If apple could do it at 25mhz or whatever, a modern amd64 machine should be able to handle it no matter the overhead of the windowing system.

great thought, but (!) I have to say from personal experience, you can bring practically any system, of any horsepower, to a crawl, with overweight layering and dispatching..

That's true. So, I would say there is an unspoken emphasis in my comment that you also need to do it "right".

Personally my X setup looks a lot like the 90s, and when people say X performs so terribly, I really don't see it. The hardware runs circles around what I used to run X on 20 years ago, and the software setup isn't really very different. But most people when they use X or think of X are thinking of a really heavy desktop environment.