Quicktip 8 (February 8, 2009) Play the base layer only in FCP
If you check the Play Base Layer Only option in the Final Cut Pro timeline pop-up menu:
then FCP will play the lowest level of realtime playable video instead of displaying the unrendered screen when there is unrendered video in the timeline. It’s a must-have when working with lots of layers that need to be rendered. In fact, I can’t think of a reason why I would ever turn it off!