> I have an X application I sometimes need to run remotely over a VPN link
> over VDSL. I have gobs of bandwidth, but the RTT sucks. The app is barely
> usable. In contrast, Windows Remote Desktop and VNC both work just fine
> over the same link. Both of the latter seem to be more of the "dumb bitmap
> plus compression" school of thought, and that seems to work pretty well
> with modern setups.
Well, this is somewhat of a strawman. The fact that X is not implemented particularly efficiently does not mean that X is not fundamentally better than dumb protocols.
If you want to compare apples to apples, see NX, the heavily-optimised implementation of the X protocol. I use it daily, and FWIW it beats crap out of the competition.