I've been having this problem where my GFX fan has stopped spinning.
I don't know when it stopped; only noticed when my performance started tanking.
After some on and off changing, I noticed that my fan does in fact spin, but somehow only when the Card's drivers are disabled.
I've updated my driver to the latest (automatically first, then manually due to the fan not working) update available, to no avail.
Any thought or reason why?
What are your temperatures when you notice the performance hit?
I have an RX 560 (takes a bit for it to heat up under load) and the fan wont spin for a few minutes even when benchmarking.
With default Radeon settings I get a board crash with a blank white screen which I assume is due to overheating when the fan will not restart. I have updated my motherboard chipset driver and BIOS and updated to Adrenaline 19.11.3. Setting "0 RPM" off and manually defining a fan profile helps until shutdown. The Radeon settings are reset to defaults on restart.