Best release of MacOS for mid-2010 iMac
I have the 27&quot; Mid-2010 iMac with High Sierra. Mine is sluggish as well but not as bad as you say yours is. My internal Fusion drive is dying and that, I believe, is why mine is so sluggish. I've had High Sierra for a long time and never experienced a problem with it. It ran fine until my drive started failing. Since you still have your original drive you may want to check its status. I use Drive DX to get the SMART status of my drives.
You may want to run EtreCheck Pro which will tell you if your machine is sluggish and possible causes and solutions. EtreCheck Pro on my machine tells me its too sluggish to be caused by software and it's probably due to a failing drive.