Chris, I'm surprised you have not seen other reports of such a slowdown. Here are just a couple of users with the same experience as mine:
https://forums.adobe.com/message/6180054
If, as in this example, I click to the turn the mask eyeball on, the mask appears in chunks, a block at time, very slowly whenever the zoom is greater than 50%. Other items (menu pulldowns, other programs, work at regular speed. Hitting ESC of course stops the process. But this is very repeatable. Here is a screenshot:
