Issue: My wife’s iMac (Retina 5K, 27-inch, 2017 macOS 10.15.7) has had a huge overall performance hit since 9.0 came out. Even cmd-tabbing to other apps has them slow, “Google Docs now has a frame rate when typing” slow and her system is now playing at 15fps in game.
I don’t know when the option was added, but in the System > Advanced options at the bottom is a pick list to choose your video output device. Auto, whatever Intel embedded GPU you have in your processor, and your discrete GPU.
I manually changed it to her Radeon card and in-game jumped to 60fps. So looks like it’s having trouble auto swapping away from the Intel embedded one. Just sitting at the character select screen and her fans were going full blast. After I changed that and logged into Ardenweald, the fans actually went down in speed. Cmd-tabbing while the game ran returned to normal performance, and using web pages/Discord/other apps returned to normal performance as well.
So pretty certain it wasn’t switching away from the embedded Intel GPU.