In any world on peaceful, mobs will rapidly attempt to spawn, spawn, then despawn(due to the world being on peaceful). This creates a substantial amount of lag, as mobs attempt to spawn every 3-20 game ticks. This happens on all 5 worlds created in 14w25b and all 5 worlds created in 1.7.9.

To reproduce:
1. Open any world
2. Set difficulty to peaceful
3. Mobs flash before they spawn and despawn

Easiest to see mobs flash on superflat(slime spawns)