Yeah X11 isn't appropriate for WAN. It is far too slow, in particular the latency for each request.Well, my experience is different. Of course, on the LAN, there's no issue. But WAN? For a long time, I had one of these 16MBit DSL lines (asymmetric, with upstream only a fraction of it, probably 1MBit or something like this). X11 wasn't usable. RDP was.
RDP is the fastest I have used so far. Which is annoying considering the network is one of UNIX's strengths
For the multi-terminal switching, there is Xpra but it seems unportable and/or a pain to set up.