With X a lot of the problem for poor performance is latency, not bandwidth.
Sending a compressed texture that is something like 1024x768 over most networks is not going to be a problem any more in a lot of cases.
A lossless 1280x800 PNG image itself is only something like 260.1KB, which will transfer over most internet connections in a fraction of a second. High quality JPEG or WebP is even smaller and compresses much faster with relatively little discernible image problems.
It's when you run into issues with applications that want to have something like a animated menu or whatever that takes 100 redraws to go from start to finish. When your on a local machine something like that is just stupidly fast and it is irrelevant. When your over the internet something that used to take 0.1 second now takes 5 due to all the time lost to latency from going back and forth 'draw' 'finished draw' 'draw again' etc etc.
When the fastest network most people had was 10 people sharing a single 10Mb/s ethernet on a single hub with all of them sharing the same collision domain... THEN that was when X networking was very troublesome in terms of Mb/s used.
Nowadays even common consumer internet connections are faster then that.
But when you have 128msec latency and it takes 2000 round trips between a server and a client to draw a new web page on your browser.... THEN that is when you run into serious performance problems. It does not matter if your sending 10's of KBs of information or your just sending 5Bs each trip it's going to create huge delays.
Your far better off just taking a image of a 1024x768 desktop at 15 FPS, sending it over a network then working on some special protocol to relay input back. (I am not sure about SPICE or ICA, but I am pretty sure that their technology is more sophisticated then just that.)
This is why people report VNC working better then X when it's obvious that in terms of actual bandwidth used X is often going to be better.
But it's not like VNC or X is even close to the state of the art. Both of them are obsolete with their own set of problems.
While people have been arguing over the merits of being able to remote access a single application over X vs a entire desktop over VNC.... the ability to remotely access your GUI over the internet has gone mainstream.
ANY PC, ANY Mac. Over your browser. Very simple to setup, relatively inexpensively, adequately secure, and good enough that the average customer can use it without pulling their hair out.
You can even do it on your iPhone or IPad....
Sure I am not going to use it and it's not suitable if you care about your security, but the networking aspect of X is far from unique or special anymore and it's performance in common situations is inadequate compared to contemporary solutions.