Do you have the MacBook Pro? I'm just wondering if you have the option to switch between the Nvidia 9400 (which is a fairly weak GPU) and another. Either way, you would be best to reduce the output size of your external monitor, as at 1440 x 900 you will be putting a lot of stress on your graphics processor. You can also try to reduce the Quality slider in the Preferences of MixEmergency. Let me know how you get on.