When caving, I've noticed that hostile mob sounds aggregate on the other side of cave walls. Mobs seem to be detect the player through solid blocks, then pool at the nearest point in a neighboring cave. This is ultimately causing lag, because some mobs are pushed by others into the blocks, suffocate, and zombies then pick up the drops and don't despawn. I've confirmed by switching to spectator mode, finding dozens of such zombies in the immediate area.