Unity using way too much (+100%) cpu when object is in scene...even when game is not running (mac)

This began happening quite suddenly and I have no idea why.

Unity will run perfectly fine if there is nothing in the scene. For example, I can insert a FirstPersonController prefab, run the game, and the player will fall through space and be able to look around.

If I insert a single plane or cube into the scene though, Unity starts freaking out. Attempting to run the game immediately causes Unity to freeze and I have to force quit. Checking Activity Monitor reveals that it's using more than 100% of my cpu. Even when the game is not running, Unity will be using 30-40% idle.

Obviously something's not right here but I can't pinpoint what. The scene file will run perfectly well on any other machine, so I'm inclined to think it's my mac, but that doesn't really give me any answers.

-Macbook Pro OS X 10.9.5 -2.8 GHz Intel Core i5 -Unity 5.5.2f1

One other thing worth noting is that when this happens, "kernel_task" also seems to be using a large amount of cpu. It seems that sometimes Unity is using ~50% and kernel_task is using ~50%.