Direct answer
Crowded collision checks can be costly even when a mob is otherwise simple. The workload rises when entities are pushed together in tight spaces.
How to check it
- Compare profiler samples with a crowded pen or entity-cramming machine active and temporarily isolated. Count colliding entities and check how often pistons, water streams or players move them.
- Record the original configuration or test conditions, capture a profile or a repeatable observation, and identify the affected world or player group.
- Make the measurement under the same player count and workload you will later use to verify the change.
What to change
Redesign the enclosure to reduce simultaneous collisions and pathing. Avoid using extreme server-wide collision limits as a first step: those can change combat, transport and contraptions.
Test animal pens, minecart mechanics and PvP knockback before keeping a collision-related setting change.
Software and version notes
Check the active version and generated configuration before copying paths or values. Different forks and mod loaders may implement this behavior differently.
This article is written for Paper / compatible servers. Configuration keys and defaults can move between versions. The source material checked for this page includes PaperMC profiling guide. If your server is Folia, Leaf, Purpur, Fabric, Forge, NeoForge, or another fork/loader, do not assume a Paper-only setting exists or behaves identically.
Verify the result
Repeat the same test with the same software build, player count and world activity. Compare the targeted symptom and any spark measurements, then check that intended game mechanics still work. If the change does not produce a reproducible improvement, revert to the recorded baseline.