I'm having a similar problem.
I bought the game on day one, a long time ago, and I was always able to play the game on high settings. However, since Patch 6, the game has been extremely slow, even with low settings, and now shows graphical glitches (various textures are red:
https://imgur.com/a/1sedw4w).
I'm frustrating, and I want to play at Grymforge.
iMac (21.5-inch, 2017)
2,3 GHz Dual-Core Intel Core i5
16 GB 2122 MHz DDR4
Intel Iris Plus Graphics 640 1536 MB
