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.

APFS is not designed for spinning rust, so that tracks.

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

I bet this is it. I had a 2018 Mac Mini with a failing drive that moved like frozen molasses, but wasn't throwing obvious errors. Before it failed, it was slow compared to an SSD, but booted up in a reasonable amount of time and ran office apps just fine, just with a little startup lag. It was bad compared to an SDD, but not intolerably slow.

If a Mac is running that slowly, there's probably a hardware issue.

Is there some reasonable way to check whether the Fusion drive is failing? Some quick searches suggest that Apple’s built in tooling doesn’t actually help much.

Sorry, I don't remember the details and it was several years ago. I do remember looking at the Console log and seeing lots of drive timeouts.

what? on a semi modern CPU and a SATA / M2 SSD?? My Vista laptop on a spinning drive took that long to boot I am pretty sure. I am flabbergasted if this is true

i5-1145g7, 16gb ddr5, nvme storage

You should be flabbergasted. It's ridiculous.