Minecraft limits how many mobs exist, but it does not limit how much work they cause. A well-built farm and a server-killing farm can produce the same items — the difference is what happens to the mobs in between.
How the cap works
The server counts mobs per world, per category — hostile, animal, ambient, water — and stops spawning that category when it hits the limit set in bukkit.yml. The counting is per world, so a farm in the Nether does not stop spawns in the Overworld.
This is why a big farm affects everyone: it fills the hostile cap with mobs in one place, and caves everywhere else go quiet. That is a gameplay complaint before it is a performance one.
Cheap versus expensive farm designs
| Design | Cost |
|---|---|
| Mobs fall and die immediately | Low. They exist for a second and never pathfind. |
| Mobs pushed by water into a killing point | Moderate. Water flow and entity movement every tick. |
| Mobs held alive in a holding cell | High. Every one of them pathfinds, forever. |
| Villager breeders and trading halls | Very high. Villager AI is among the most expensive in the game. |
| Item elevators using entities | High, and usually replaceable with hoppers or a chute. |
The classic report is «the server lags at 8pm». That is often one player logging in near a farm, loading its chunks, and starting several thousand entity ticks that were not happening a minute earlier. Look at who is online when it starts, not just at what the profiler says.
Finding the farm
- Count entities by chunk
/spark tpstells you there is a problem; a plugin that lists entity counts per chunk tells you where. Almost every performance plugin has this command. - Go and look
Fly to the chunk. A profiler can tell you that entity ticking is expensive; only your eyes tell you it is a villager hall with four hundred residents.
- Talk to the owner before breaking anything
A farm is somebody's project. «This design costs the server a lot, here is one that produces the same and does not» keeps a player; removing it silently does not.
entity-activation-range decides how close a player must be for a mob to think. Cutting it means the same mobs exist but stop pathfinding when nobody is near — the farm keeps working and the cost drops.