Although the site doesn't list specific symptoms, it acknowledges a possible OS X Lion conflict between My Living Desktop and System Preferences items that similarly interact with "QuickTime movie playing." It cites (which I do have installed) as one potential culprit. My Living Desktop's recommendation is to uninstall Flip4Mac. I never got around to testing this, so I can't confirm if it works or not. But if you are experiencing any similar slowdowns, it's worth a try.