I've had two instances of chunks turning into black holes that then generate a large number of mobs near them that lag the game. First happened in the Grey Mountains and spawned more birds than Hitchcock's "The Birds". Second time I was in Gondor and using World edit to move a section of blocks when the hole appeared behind me, and not in the blocks I was moving. Now my town is infested with Gondorian soldiers and butterflies. I don't know if WorldEdit caused it the first time but this is the first instance of this glitch happening in the world i'm using, and I've been playing around this creative save for 2 years-- really don't want to have to get rid of it. Help?
Yes, those are corrupted chunks. Usually they're caused by improperly saving the game such as pressing the exit button before leaving the world or a sudden loss of power to the computer. I've never heard of world edit causing this, but I suppose it's possible. When a mob walks into one of those corrupted chunks, the game thinks it disappears and it spawns another one to replace it, but in reality the first one hasn't disappeared and thus you get duplicated mobs.
Anyway, on to fixing the problem. If you're good with computers and technically inclined, I could walk you through how to fix it yourself. If not, it'd probably be easier to just send me your world (or part of it). If you want to send part of your world, here is a list of the files I would need:
.minecraft/saves/<Name of your world>/MiddleEarth/region/67.138.mca
.minecraft/saves/<Name of your world>/MiddleEarth/region/67.139.mca
.minecraft/saves/<Name of your world>/MiddleEarth/region/68.138.mca
.minecraft/saves/<Name of your world>/MiddleEarth/region/68.139.mca
.minecraft/saves/<Name of your world>/MiddleEarth/region/117.-42.mca
Once those chunks are fixed, you should be able to leave the area and return to have the mobs despawn naturally. If you want me to remove the mobs as well, I would need one more file: