I found a partial workaround on Creative Cow, but I think this is a bug in the multicam feature.
First, I deleted all the unnecessary audio tracks in the original sequence (I only need one, of course). Then I turned off the audio for this track, copied the track, and pasted it in the nested sequence. Performance is back up to snuff, except that I am still getting occasional delays from PP "rendering required files."
It's my understanding that this should not be happening. The yellow bar does not turn green after the render, there's no obvious reason why anything needs to render at all. The original sequence plays just fine on it's own with all the tracks on, even the audio that I was able to delete. Once the edits are made in the nested sequence from the multitrack window, it should just be referencing the original sequence, turning on and off video tracks. Seems like no big deal in terms of processing power. And it doesn't look like the effects are causing the render, as these are GPU things and the original sequence plays all three tracks smoothly effects and all,
And lastly, why would I have no problems at all when I edited these sequences in the first session, then have all sorts of issues when the project was opened at a later date. BUGS!
This is one of the problems I have had with my 12 Core Mac Pro that I never had the chance to report yet.