Thanks Paul. All of those are set/have been set. One 4K (using built-in display and even another 1080p monitor) is fine ... two 4Ks (using nothing else, just the external displays with MacBook in clamshell mode) does not work well. It's starved of something.
ETA: I've switched to "scaled" mode for graphics ... not as sharp and a bit hard on the eyes, but workable and much smoother performance. I'll have to live with this until a fix is released.
ETA2: Nevermind. Scaled mode made my brain hurt pretty quickly.
Last edited: Jan 20, 2017