Part of it is toolkits throwing sometimes multiple megabyte bitmaps to draw, which starts to have problems the moment you don't have a zerocopy, gpu accelerated method to draw them.

The old core protocol approach extensively used optimized graphic operations on the server side, with clients sending things like "draw me a rectangle/fill a rectangle/draw a bunch of lines" etc. - today you're going to get pretty big bitmap (especially with high dpi).

It's the same problem that mobile devices faced, and is related to a lot of issues on how android devices were "janky" (and related to the various hacks that Apple did to make sure your application wasn't capable of overstressing the early iPhones - cause just displaying basic UI was close to doing that.)