Direct answer

The published Leaf 1.21.1 config lists async.async-mob-spawning.enabled. It may reduce some tick work, but spawned mobs still obey mob caps, difficulty, gamerules and world mechanics.

How to check it

  1. Check the exact server build and generated leaf-global.yml before using the older documented toggle.
  2. Record mob count and farm yield under identical difficulty, player positions and simulation distance.
  3. Check the log for worker errors and confirm that any spawning problem is not caused by mob caps or another plugin.

What to change

async:
  async-mob-spawning:
    enabled: true
  1. On a compatible test build, compare the same farm with async-mob-spawning enabled and disabled; make no concurrent spawn-cap changes.
  2. Do not try to fix a non-spawning farm simply by enabling this performance option; investigate light, spawn space, caps and player distances first.

Software and version notes

Compatibility and safety

The reference page is based on Leaf 1.21.1. Later versions may rename or remove the setting, so generated config and matching release notes take precedence.

Verify the result

Measure spawn counts per minute, CPU load and log errors. Keep the prior config ready if gameplay or compatibility changes.

References