So I have in i7 8-core processor, I have cores 6 and 7 solely dedicated to running the overworld and caves servers respectively. But since this game doesn't allow me to run a server on multiple cores, it still means the overworld is limited to relying on 1/8th of my total processing power. It worked fine at first but it's gotten extremely bad lately, with players experiencing slowdowns and rubberbanding that makes the game absolutely unplayable.
I am fairly certain bandwidth is not the issue as 1) all speedtests and ping tests come back normal and 2) I also experience extreme slowdowns and rubberbanding as the host.
CPU being the bottleneck seems to be very likely. I have managed to make things better by installing a mod that periodically cleans up clutter on the ground and limits active mobs / item spawns. I've also run a command that deletes all liftable, non-container and non-essential items on the ground. It's not quite solved yet.
Has anyone run into that issue and found a solution that does not involve restarting on a smaller map? Ours is huge and 150 days old, started getting unplayable around day 135.