I am amazed. They finally actually made the program better. It's been years. I've already switched over to Resolve but I thought, hey...lets play in 15 and see if the performance has gotten any better. And low and behold - it played my 4K clips easily. Which it never did before. (I'm on a 10 core iMac Pro with VERY FAST SSD's). Then I put a chromakey clip on top of the 4K clip and it still played. And then some color correction and one more layer and it still played! Yahoo! It's only taken them years to wake up. Thank you adobe...but what took you so long? And don't stop now, I may even come back to you from Resolve...maybe.