The RAM preview is far from perfect, not the best for a complete review. Usually small segments BUT, your system is a main contributor on that, like how much RAM you have available but also the type effects. Some effects suck up resources so bad, you almost can't do a preview without turning them off. Like motion blur for example sucks the life out of things so if there are parts that have major effects that you do not need to really see as part of your preview, try turning them off.
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