I have gnembom's enderman farm on my survival server, and this thing produces more xp than you can collect, so I have lots of xp build ups. the problem is that these xp orbs are very laggy. once I kill about 200 endermen (~1 min of using the farm) i start to drop frames and server tickrate. 45 seconds later I dropped down to 1 frame per 3 seconds and the server tickrate dropped significantly too. the fix I'd suggest is to have xp orbs stack, just like all entities, or make an option where rendering xp orbs can be turned off. (btw there were about 750 entities. 70 endermen, some some ender pearls, and the rest xp orbs, so probs 500 individual xp orbs)