Weird FPS thing....

So here's a weird thing that's going on at the moment with my mac client.

When the game starts, My FPS will be 27-37 consistent. But if I tab out of the game to desktop, and then tab back in again, my FPS becomes a consistent 47-64 FPS. If I tab back out again and back in, it'll drop back to between 20-40something.

The FPS increase doesn't carry over to a new game.