DimensionalThreading VS VanillaFix

The Dimensional Threading mod offers this on 1.16+ fabric servers

You don't need to redesign the server. You can just use these mods.

Dimthreads may cause some issues, though seemingly none that are similar to yours https://github.com/WearBlackAllDay/DimensionalThreading/issues. Also make sure you configured it to use more than 3 threads since you have additional dimension mods (Aether);

Starlight performs substantially better than phosphor and greatly speeds up terrain generation. Krypton for networking performance. C2ME or tic-tacs (both aim to do the same thing and are not mutually compatible) greatly improve world loading and terrain generation but are experimental and, in my experience, not compatible with a lot of other mods. Dimensional Threading puts each dimension on a separate thread, but also tends to not be very compatible with other mods. FerriteCore aims to reduce ram usage. This page has a whole list of mods.

(Experimental) Dimensional Threading ( WearBlackAllDay/DimensionalThreading )

SOLVED: Thanks for the suggestion! I just figured out the problem. I had the experimental mod called Dimensional Threading installed on the server https://github.com/WearBlackAllDay/DimensionalThreading.

VanillaFix DimensionalDevelopment/VanillaFix