Planar tracks impossible to preview

I'm not sure if this is a bug or I'm doing something wrong. I have a shot that I did a planar track on and added a mural. I've used both the built in planar tracker as well as Mocha Pro. I'm able to complete the composite in almost real-time, but when I try to play the shot, Fusion (beta 4 and 5) just locks up and won't play. If I hit Stop, I can continue to work and do other things. I can also click through the timeline and the preview updates. The issue is only when I try to play. Adding a Saver node and rendering works just fine. Any ideas?