iMac 2020 - Downgrade OS

Hi, My father has what I think is a 2020 Intel iMac 4K, and it is currently running Monterey, but it is very very slow. I have checked to see what is running on it, and deleted a lot of rubbish that started up at boot, but this did little to improve things. I read that Monterey was the first OS to be "optimized for Apple Silicon" which I assume was the M1. Is this why it is slow? I can't remember which OS it originally shipped with, but I was thinking of rolling it back a version to 11.x... Read more