Structures Generating in any biome and World-Loading lag

I was making single player servers and I noticed I'm getting two or three times more lag when generating new land than before, and also I found a Witch Hut between an Extreme Hills and Plains biome, there was not a Swamp nor Dark Oak Forest near by. I am not sure Witch Huts generate in Dark Oak Forests, but I have seen them since 1.7 came out.