Oh, wait a minute... there seems to be some confusion. Sibelius runs just fine on its own. It's just when I have a pretty hefty VI template in Logic _on the same machine_, with Sibelius playing the score that the gagging begins. Perhaps this is not recommended (i.e., Sibelius as sequencer on same machine as Logic VI host), but I'm still not too impressed. Playback in Sibelius is just basic MIDI stuff, so it shouldn't bring the audio down system-wide. But the score following/drawing is pretty smooth, which can't be done without chewing up resources.
Of note, though, is the fact that DP does quite a bit better. Not perfect, but better.
So maybe Logic's the pig! (Or maybe it just doesn't play well with the other kids.) BTW, has anybody used Logic Express for hosting VIs?
J.