I have a Gigabyte RX580 8GB, and after updating to the newest drivers, I get 80 FPS in CS:GO instead of the usual 300+. I get 14 FPS in Minecraft instead of the 120+.
I have an Intel 6600k CPU, 16GB of RAM.
Roll back the driver, had the same issue, 20.4.2 solves it.
I have a RX 570 8GB, 20.4.2 and 20.5 drivers are ok, but I use a 2019 enterprise driver. 2019 drivers are faster than the newer ones. Just roll back.
Are you using Radeon Overlay? If so it can cause a performance drop, this is listed as a Known Issue in the release notes. https://www.amd.com/en/support/kb/release-notes/rn-rad-win-20-5-1
As mentioned above, rolling back the driver should resolve the issue.
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