Well the counter example of that is the Raspberry Pi, it runs X11 poorly while having more capacity and much more performance than the 486 and original Pentium (which are '90's machines). I've been working with a 'Pandaboard' which is an OMAP 4460 system, and its challenged too.

So perhaps we have a different definition of 'is fine.' :-) Have you run X11 on an RpI ? did you think that was fine?

I have never seen a 486 running X11 at 1080p. Now days there are tons of layers of crap on top of X11.

In 1992 (or 1993 ?), I had a 486DX33 with 16 Mb of memory and 120Mb HD running linux 0.99pl12 at resolution 1152x900. It was very confortable, no problem for compiling kernel while playing tetris.

I haven't run it on one, but were you using Gnome or something more modest like "twm"?

I run the default window manager which is really basic on Raspbian "Wheezy". In my application I ssh tunnel to a 'big' machine, startup Firefox and have it display full screen on the RaspPi (its a really cheap 'dashboard' machine). One window, over a gigabit link, updates, mouse movements, keyboard response, with multi-second latency. Not fun at all.

You think that's X11 and not _Firefox_?

Which had a very different architecture than current smartphones or PCs. There was no OpenGL in 1990...

And yet X11 works just fine (network transparency included) on my N900 with 256MB of RAM . . .