Direct answer

In spigot.yml, merge-radius.exp affects the merging of XP orbs when they initially spawn. It does not control all ordinary XP merging after spawning. Tune it only if orb creation/count is a measured part of lag.

How to check it

  1. Reproduce XP-heavy activity with a known number of farms or players and record MSPT, XP-orb entity counts and pickup behavior.
  2. Inspect the merge-radius.exp entry under the world settings in spigot.yml; check for a world-specific override.
  3. Use spark to see whether entity-related work is important before assuming this setting fixes a slow server.

What to change

The documented reference shows exp: -1, which disables this extra initial-spawn merging. A positive value sets the distance in blocks for that initial merge. Test a small change on a backup/staging world and confirm XP pickup still feels correct.

# spigot.yml:
world-settings:
  default:
    merge-radius:
      exp: -1  # disable extra merging on initial spawn
      item: 0.5

Do not copy arbitrary values from old optimization lists. This setting differs from merge-radius.item, which affects dropped items.

Software and version notes

These keys and their documented semantics are for Spigot-based configuration as described by PaperMC. The code location and behavior may differ on Folia, Leaf, Purpur and modded platforms; verify the actual server version.

Verify the result

Compare orb counts and XP pickup in the same farm scenario, confirm experience is not lost and check that tick cost actually drops. Restore the old value if XP behavior is surprising.

References