I am here with the exact same problem. Typing is laggy, I look at the spinning rainbow ball multiple times a day now when I have never seen it come up on this computer before. I have a 21.5” iMac mid 2017 Retina 4K, i5 with 8GB ram that has never struggled to do anything which now takes a decade to even turn on and then once it turns on, further time to even become usable. Appalling this hasn’t been fixed