It’s possible that it doesn’t play well with JS garbage collection, since it recreates the whole UI structure for every frame (which tends to not to be an issue in the languages immediate-mode is usually employed).
But yes it’s a bit more akin to game renderings than web rendering. Which can be totally fine if done well.
In 1GB I could probably fit all the buffers to double-buffer all the TUIs in a whole country. Well, maybe not. But it's likely not that far off.
All those CPU to render this effect
(to be read with the Unreal Tournament announcer voices, see https://www.youtube.com/watch?v=MwxjYFqP35A )
At least that is what Moonlight client claims.
