Attacking zombies under certain circumstances causes frequent slowdown and crashing

Essentially the situation is thus: an artificial village has been created in the form of a towering multi-level building, currently containing approximately 5 villagers; some cured from zombification, others subsequently spawned as children. The building is completely enclosed, with a fence + gate at the front, no methods of entry provided to zombies or other mobs (bar, obviously) spiders.

Attacking zombies from behind the fence tends to cause a lot of lag, and can frequently cause a total lockup, requiring a force-quit, which produces this crash log.

Steps to Reproduce:
1. Create a village (or find an existing one).
2. Enclose the village completely, permitting no spawn of zombies or other creatures within its walls, and allowing no external access except via methods only controllable by the player (e.g., fence gates, piston doors, iron doors, etc.) – including fencing off any nearby high ledges where zombies may dive from to reach the villagers.
3. Wait until nightfall.
4. On nights where large numbers of zombies will spawn (more than 4-5 zombies), attack the zombies from within the enclosed village.
5. With a bit of luck, you should notice some heavy lag, which may be alleviated if a zombie capable of spawning reinforcements manages to get a reinforcement spawn within the village.
6. If no reinforcements are spawned, the game may freeze up completely, necessitating a force-close.