Why does the game stutter or run slowly the first time I play or after I upgrade OS X?
If you are using an Nvidia graphics card, you may notice low frame rates and stuttering for the first few minutes the first time you play the game. This is because the game is loading new areas for the first time.
As you play, information specific to your Mac is generated and stored so that it can be loaded more quickly the next time it is needed. This means that the game's performance will improve as you play, and will improve significantly the next time you launch the game.
This information may be deleted when you upgrade OS X, so you may experience the same temporary problem immediately after an upgrade.