It's like a trick question; I find that the software/firmware updates effect the graphical interface more than anything else including higher model numbers (which you would assume contain better parts). I've had receivers that were smoking fast then a software update happens overnight & you are back to turtle speed. About the only thing I can say that always speeds up the GI is changing the scrolling effects to off: Setup>Setup>Display>Preferences> & selecting Scrolling effects to off. I've gown used to the receivers & like the layout but speed appears to not be high on DTV's list of "To Do's".
edit: Would gladly pay a $100 premium for a unit that guaranteed fast GI; some of us are just channel changing freaks.