When a large swarm of zombies is after a villager they cannot reach, it causes severe lag. Entities do not behave correctly i.e. zombies do not take damage when you try to hit them. This is likely due to ticks taking longer than normal to complete because mobs move slowly and day and night does not cycle at the proper speed. In some cases I can even see the clock dial go backwards. The issue is resolved once the villager is dead or becomes reachable, and it does not occur if the zombies are not swarming. There is no fps drop, just typical lag. Here is a link to a video of my experience. http://youtu.be/hEcINIdVEtI

What I expected to happen was...:
No lag

What actually happened ...:
Severe lag. Entities do not react.

Steps to Reproduce:
1. Surround villager with fence so zombies can't reach it.
2. Spawn zombies far from the villager so that they will swarm after it. Swarming doesn't occur as much when they are spawned nearby.
3. Attempt to hit zombies, throw splash potions, notice how time slows and experience the general lagginess.
4. Kill or release villager and notice the lag ceases.