X11's main flaw is that it's supposed to be inefficient. It might be, but I've never noticed any significant difference between user interface performance on Ubuntu vs Windows or Mac.
Try tunneling X over a VPN connection from home to work. It sucks for most applications. Apparently there is a lot of "back and forth" between the client and server where latency is multiplied by orders of magnitude.
This was years ago, but I installed NoMachine's NX server (version 3.x) and things worked very smoothly, almost as fast as being local. NX eliminates a lot of the "back and forth" in "X" which demonstrates that you don't really need that extra overhead in the first place.
However, NoMachine seems to have screwed the pooch with version 4.x in that they only support a virtual desktop (like VNC), which makes it MUCH less desirable to me.
So, yeah, I am perfectly OK with something to replace X if it is a lighter weight protocol and can natively support remote sessions without having to jump through hoops to get usable performance.