hi Richard,
you may have already fixed this but,
I've noticed on all my systems that playback performance is hugely reduced when both outputs are in full screen mode.
I am using both apple intermediate and photo JPEG codecs.
I have a TH2G on DVI2 and a local monitor on DVI1. Playing back one movie on a layer, outputting through the THTG, if I have screen 1 in full screen with the HUD up, playback is noticabley reduced. Taking screen 1 out of full screen corrects this.

Regards
Paul