> Fab stated that it ran the HD movie with no problems

Yes, and he stated even some more, albeit not relevant for MorphOS:

"OSX is jut not able to do it because of the additional overhead in the graphic system (or whatever)."
http://amigaworld.net/modules/newbb/viewtopic.php?topic_id=32428&forum=33#580833

Is it maybe that video replay doesn't benefit from SMP and thus can't leverage the second CPU core to make up for the additional overhead?