Bat Lag World

Me and my friends were making a map and in some places where redstone was, it lagged. When I went to mcedit to look and maybe see the problem, I saw red blocks When I went in the server to check what it was, it was a bat. When I removed all entities in that area it stopped lagging but when they spawned again it started lagging again. It may be because of the more you are in an area the more mobs spawn and so on but the lag stopped after I removed entities which were bats. Even when I explore a cave, bats spawn and that area starts lagging. My friends also experience lag over bats. I meant was 1-2 bats lag the chunk. 25 cows don't lag the chunk so what is the problem?