Strangest thing. I have a maxed out iMac. I have edited a lot of 8k projects just fine. I have an HD sequence with a mix of 4k and HD footage. (The clips where provided to me so I don't know what they were shot on) If I click the play head around the timeline it has a major delay before it loads the clips. Even if they are rendered. It seems like the 4k 24-bit footage is the problem but I'm not sure. In an HD timeline with just the 4k footage, I don't have any problems. In the HD mixed footage sequence, it has a delay when I hit the space bar before it will play.
I just loaded an old HD sequence project with RED 8k footage and have zero problems. I cleaned the unused media casch database to see if that would help but didn't do anything.