Tickspeed Bug (16w06a)

What I expected to happen was...:
I expected minecraft to run the same no matter what the gamerules are set to.

What actually happened was...:
When the "doMobSpawning" gamerule is set to true, the minecraft tickspeed slows down tremendously.

Steps to Reproduce:
1. Enter the chat.
2. Set the difficulty to easy, normal, or hard.
2. type "/gamerule doMobSpawning true"
3. Activate and deactivate any piston repeatedly to visually see the tickspeed plummet.