I have been looking for ghast cage designs for a tree farm, after finding one I built it and tested it, only to find that when the ghast got aggroed the game's lag immediately plummeted.

I do not know the cause of this, however the ghast is in a minecart and is attacking a snow golem. The fireball gets redirected by a slime block.

To recreate:

1. Get a ghast to attack at a snow golem (do not know if this applies to other mobs)
2. There should be some noticable lag.