I'm currently using the mod Apotheosis which adds a lot of custom enchants with a much higher enchantment cap. Included in the mod is a way to make a huge amount of XP with a sword. When someone does this on the server it causes the server to lag behind and eventually crash due to ticks not being registered for over a minute. I'm not sure if it's a problem with the mod since I'm sure it works in Vanilla, but I thought I would mention that it may cause a lot of strain on the server when running XP mods.

Not an issue with my mod, apotheosis should handle the huge amounts of XP better (as in don't spawn so much)