True, applications aren't requesting that the windowing system draw a line segment from A to B, and so on, but in practice X11 applications aren't doing that either.
There is a middle ground in between basic line-drawing primitives and bitmap scraping. RDP does typically work at a higher level than screen scraping, which is why nix's example of scrolling a text editor sounds highly suspect - I've just tried connecting to a Windows desktop (via whatever mechanism Remmina uses for rdp; I guess it wraps rdesktop), opening a large-ish file in gVim, and scrolling both casually line-by-line and madly by mashing page down. It's just as fast and responsive as running vim in an xterm over the same link (ping ~45ms), and in fact seems to have slightly less input lag. Notably, looking at the data transferred, RDP is using a fair bit *less* bandwidth.
The point is that VNC is not comparable to this. It very clearly has to resend large amounts of data for even single line scrolls; it doesn't work in the same way at all.
With any luck Wayland's remoting will sit at a similar point to RDP on the primitives<->scraping spectrum, and everything will be rainbows and unicorns.
 That is, 'ssh -X <host> xterm vim'. For a more direct comparison, I did try 'ssh -X <host> gvim', but that really wasn't a fair comparison since X is useless over WAN links. The bandwidth requirement was higher still, and the input latency was so high that nobody could seriously consider it usable in any but the most desperate of circumstances. If you are only ever moving line-by-line, then it would win over VNC, but if you ever want to scroll a whole screenful it takes a similar amount of time.
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