Extremely Laggy even on Singleplayer

I play Minecraft on an iMac. It is usually runs very smoothly, with no lag, even on packed servers. But in every snapshot since 1.7.4, the game has been lagging every time I look around. I have put the graphic setting on the lowest, but it still happens. I normally play with all the graphics at their best, and it is usually fine, but not for these snapshots. Even when I am not running any other apps, it still is laggy. I don't know what to do...