Same problem over here! I'm really getting crazy. I work a lot with Adobe Creative Suite. Normally I could have three programs open at the same time. Now I'm seeing the beach ball every 3 min with just one program open.
iMac (Retina 5K, 27-inch, 2017)
4,2 GHz Quad-Core Intel Core i7
32 GB 2400 MHz DDR4
Radeon Pro 580 8 GB