This screenshot shows the last few GB5 Metal and OpenCL scores for my PowerColor Red Devil RX 580 card. These scores are for my iMac1 system running Catalina and Big Sur. Not any significant difference between the way the card works in both OS's. I've not installed Monterey yet, as I need to change my SMBIOS to install the newest OS and to say I am reluctant to do so would be an understatement.
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Here is a link to the top Big Sur Metal score, which as you said is significantly better than your Monterey score -
3637319 Not just the combined score but a lot of the individual scores are much better in Big Sur.
The question then is what background processes are happening to use the GPU's processing while you use Monterey?
Or is it an issue with the AMD drivers for the Polaris 20 GPU's in Monterey?