What laptop is this?
There are a couple potential reasons for this, and all or none may apply to you:
1. (only applies if you have rmbp)You have applied the EFI update that comes with ML which throttles GPU/CPU performance more aggressively. Only you'd know if it's been updated.
Unfortunately there is no permanent fix for this, and no way to downgrade. Some forum users have reported that a SMC reset temporarily fixes the problem. It will probably come back though.
2. The game world maps that you interact with after you reached level 6 are more complex/difficult for the GPU to render.
I have borderlands 2, and I noticed that my performance dropped a little once I actually got into the game past the tutorial area, as I encountered more enemies at once and more complex areas... these were logical drops in performance though. If you're noticing that the SAME area you were playing up til level 6 is no longer playable, then it's probably the throttling to blame.