I noticed in another thread someone had said that the fps in their games dropped considerably by using F15 compared to F14.
I had suggested using a seperate X server so the graphics chip is not rendering the desktop and game at the same time. There was no response so I am not sure if ti helped or not< but...
In theory you would just have to type.
xinit /usr/bin/nexuiz-glx -- :1
In the above example just change the path to the minecraft excutable. This should start a second X server and only render what is on screen. Your desktop would no longer be rendered at the same time in the background with the game. ctrl+alt+F2 or the proper corresponding F(#) should allow you to flip back to your desktop and gaming desktop while the game is running.
If this improves or greatly improves the performance we then know it is the graphical rendering of the desktop that is effecting the games so much. If not we need to look at something else that might be causing it.