I’ve helped someone with a rather clean iMac, circa 2019, still supported by Apple. Forget 6 minutes — you can spend a full hour from boot to giving up trying to get anything done.

I think that Apple has gotten so used to having fast storage in their machines that the newer OSes basically don’t work on spinning rust.

these iMacs have horrible Fusion drives (128GB SSD + 1TB HDD combo) iirc that fail often. Have you looked into that?