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Download and run Xbench, but unselect "Thread Test" in their dialog box. For some reason Xbench hangs on Lion so the fix for it is to unselect thread test.

This worked for me.

I just registered to say THANK YOU, this worked perfectly.

My early 2011 15" MBP OSX Mountain Lion just started lagging in mission control and when switching desktops last night. Couldn't be happier that it's fixed.

Simoquasimo, what is dGPU and how has xbench triggering it resolved the problem?
 
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