Entityopt

EntityOpt is a server-side Fabric mod for Minecraft 1.21.11 that helps cut down on entity lag. It works by putting mobs to sleep when no players are around, so your server isn't wasting time processing hundreds of entities that nobody can even see. Mobs near players behave completely normally; it's only the ones off in the distance that get throttled. It also comes with a few handy commands to spot where entities are clumping up, check live stats, and ”purge” (aka get rid of) them if things get out of hand. Named mobs, leashed animals, and anything being ridden are always left alone.

Dormancy is the system that handles the lag from mobs. Entities within 48 blocks of a player run as normal. Between 48 and 96 blocks, they move less frequently, and beyond 96 blocks, they go fully dormant and barely do anything at all. As soon as a player gets close again, they wake up instantly and resume normal behaviour.

Commands:
/entityopt hotspots - shows where entities are clumping up
/entityopt stats - shows live entity counts and how many are dormant
/entityopt dormancy - toggles the dormancy system on or off
/entityopt purge [radius] - removes all nearby entities within the given radius

This mod is tested on server-side only, and has only been tested on 1.21.11. I'll add more versions and other server platforms later, but for now, just enjoy Fabric. It works with other mods and is currently used on 1 server. If you plan to use this on your server, please dm me on Discord(Pheinmanski) so I can update this number.