Q:

Best release of MacOS for mid-2010 iMac

I have a mid-2010 iMac that I haven't used for a while. I wiped the disk and installed High Sierra (10.13.6). I don't remember the previous release of MacOS it was running with. It has a 3.2GHz i3, 1T HDD and 12G of RAM.
It's incredibly sluggish, clicking or typing will often result in the spinning wheel for a few seconds before anything happens. I've installed nothing on it beyond what High Sierra does by default.
My intention for this Mac is to use is for web browsing, writing, watching videos and music creation (with Garageband or other DAW).I have no intention of changing the HDD to an SSD - I don't think it's worth the cost and I have 2019 iMac that I have access to. This is just a secondary computer I'd like to have in my house.
Would an older release of MacOS fix this sluggishness? What release of MacOS would you recommend?

A:

I would plug in an external SSD - those machines continue to run as fast at work as they were the day we purchased them with one exception: storage degradation

Fusion drives are likely done with fast writes unless you have it powered off got 8 of the 10 years it served you.
HDD at 10 years are just going to need an erase and probably retirement, anyhow.

Get an SSD or external drive over thunderbolt or FireWire and install the newest OS it can run. Then migrate all apps, settings, data you care to get off the internal drive and then erase the internal drive so you don't see it as a drive on the desktop is my advice.

