I have a 2013 Mac Pro trashcan, 6 core, and the strangest thing happens: It can play unrendered clips well, even with a fair amount of effects applied. But it sometimes refuses to play the rendered clips. This is of course opposite from my old Final Cut Pro 7. In Final Cut you might not be able to play a clip after you've added a few effects but after you rendered them it didn't matter how many effects you applied--it just played. This does happen more when I have a few timelines open and if some of the clips have a lot of effects Does Premiere "render" files in a different way?

This is actually really scary :(