I'm sure this is nonsense, but I don't know enough to know why it doesn't work, I just wish it was something that did work.
The thought is that you select a desired area of chunks to save locally for fast loading/rendering (let's say I want an area of 128x256 chunks / 4x8 regions / 2048x4096x384 blocks), so you can fly around there at Mach 10 with your modded Elytra with no loading issues due to the server struggling.
The goal being to reduce server load as the only updates the server gets or sends should be when it deviates from what you have saved, as in:
- When you or a mob in your view change the terrain from what was already saved on your end, either by yours or a mob's destruction/construction.
- When another player or a mob in their view change the terrain, and the server saves changes to send to you when you connect again, or continuously while you are connected.
The server would need to know which chunks you have saved locally and it would need a copy of your saved terrain state, I assume.
(And of course, if it's a crowded server or just a frequented area this wouldn't even help if it worked, so at best it would be situational. If it worked, this would probably only be useful in situations like mine, with a small server with friends, where we might have a favorite area we like to return to, and would like see the entirety of while roaming.)