I have experienced similar situation where the frame playback drops to 4 or 6 fps, but in tiger it was running at 60fps.
I also have some comps where the interactivity inside QC becomes a frustration because I have to wait until QC decides to let me do the next move.
Also, I have noticed that inside QC I can do a "Tiger runtime" test, and then the composition seems to play more fluid, but when I am adjusting or editing the comp, it is very slow to the point that I need to restart the application and then it becomes "just a little bit faster"
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