I have an entity that I spawn by the thousands, but there's a massive bottleneck because whenever the position of these entities changes it sends a packet to the client. The problem with this is that at the level of thousands sending all those packets kills any performance. I cant just set the entity to be still on the server because of tracking range and bounding boxes. Is there like a way I could tell the server to Not send any position packets for this entity?

 

The entity can handle itself on client so all these packets are useless 

What does entity actually do? It is impossible that is can in any way "handle itself" - The rule says: "Client always lies!" - you can't expect for few clients to be handling their own entities without server.

 

Only one option is that the entities in question are PARTICLES, and particles and CLIENT SIDE - and yes - you spawn them on client, they can handle themselves and don't need server.

No its a child entity, theres a parent entity that is receiving updates and that sets the children's positions based on where it remembers them. These dont need anything, so how can I set them to not send position packets?

Look at:

EntityRegistry.registerModEntity(...)

 

You can set update freq to very high.

You might also want to look into PlayerEvent.StartTracking - might help somehow, but doubt it.

 

EDIT

Other than that - EntityTracker is the thing that handles tracking if given player needs to get entity's data. You can alter it with asm (might be possible wiht reflection - replace object).

Do these child entities need hitboxes and stuff, or are they more just a visual effect (like a something flying around a parent entity)? If it is mostly visual, then I suggest you have just one entity (the parent) and just render the others (only on the client). The parent entity would have fields to keep track of the child entities, and the server would only keep track of the parent entity. If the server needed to know or control the child entity positions, you could have a custom packet that updates them all at once.

It really sounds like you have something acting like a particle not a full entity.  I would suggest not creating a full entity for them.  Just create a client side render mechanism.