Opening large sessions - beachball

[1m1]

I'm doing a job where I am recording several songs back to back on the same timeline in the same session. It's about 72 tracks at a time, and the session has about 20 pieces in succession that are about 30 seconds each. When recording the system works great, no sluggishness at all. I can record, stop, then record again, stop, play, etc all without the session hesitating. The issue is when I close the session and then reopen it. It takes about a minute before I can get it to play... this happens both with the disk cache set to normal, and I've tried both 20GB and 10GB settings. I'm not sure what the computer is busy doing during this minute. The task window doesn't show anything going on.

Any suggestions on how to get it to be responsive when opening a large session?

I'm running 10.3.3 on a 2010 Mac Pro with 40GB of RAM. HD3 system.