I've created an effect in the timeline which uses nine video tracks (V2 - V10), so it's pretty choppy when playing back. Under these tracks (in V1) is the video this effect is playing over, so to try and make this section of the timeline playable in real time, I switched V1 off and went to do a video mixdown of tracks V2 - V10, assuming this would create transparency underneath them...
Pretty surprised to find out that video mixdown doesn't support alpha channels, it just flattens it all out... An equivalent feature in another editor I use (Edius 8) does support this kind of thing.
Just an idea... unless it's a feature buried elsewhere in Avid?
I'm using version 8.
(Note, I did try a luma key on the mixed down video, but it absorbed the shadows on the text within the effect)