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When I use multiplane FX and switch on preview, it will barely run.
That is, when I try to play using preview, it is really, really slow to the point of unusable. I end up having to switch it off and hope for the best, render the move and then if I need to make changes, go back and do that and render again. Is it normal that preview does not function? Or barely functions?
I have a very fast iMac and the scene has about 12 layers. I am doing a hold and then a zoom in with a multiplane effect. Nothing too complicated.

12 Layers is a lot for smooth playback in any software. I do my multiplane setups in Moho (faster setup), but previewing it is slow. I can speed it up a bit with reducing the view, but for testing a smooth run I have to render to video.