A couple of thoughts:
I wonder if there is some sort of ancient issue about having an interface more than 4096 pixels across? If you resize the UI to say 75% or less of the screen, does make a difference?
If you Get Info on Animate there is an option to open in Low Resolution. I just did a test on my MacBook Pro, and with a large stage 60 fps, pressing Return to have it run through the timeline over 240 frames, it took 5 seconds in low resolution on both external Cinema Display and internal Retina. Not quite the 4 seconds it should take. In high resolution it only took 5.5 seconds, so for my MacBook Pro it's not a lot in it. But then I only that many pixels across both of my screens combined!
If trying the low res trick solves performance for working, you might fret about not getting the most out of your 5K screen, but at least you can get some work done.
Let me know what you see, including the 75% of the screen for the UI test.