MacOS High Sierra 10.13.6

Sorry, that is the highest possible. See the following from Identify your iMac model - Apple Support

iMac (27-inch, Mid 2011)

Model Identifier: iMac12,2

Part Number: MC813xx/A, MC814xx/A

Latest compatible operating system: macOS High Sierra 10.13.6

ocholoco Said:

"MacOS High Sierra 10.13.6: Ok, thanks. My iMac just started running very slowly this weekend. What would you recommend I do first? Thanks!"

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Some thoughts...

Troubleshooting a Lagging Mac:

A. Try Safe Mode:

In Safe Mode, login items do not run, and corrupt system files(i.e. caches) are intentionally reset as a way to fix issues

1. Boot: in to Safe Mode: [Hold Down: shift key upon boot].
2. Log in: as same user
3. Wait: 30 seconds
4. Restart; your Mac
5. Log in: as same user

B. See if your Hard drive is Failing:

What to Try:

• Verify your Drive: Repair your Disk in Disk Utility - Apple Support
• Go Here: If your Mac Starts Up to an Apple Logo or Progress Bar - Apple Support
• Perform the following in Recovery Mode:

1. Boot: into Recovery Mode. Go Here:  How to Use Safe Mode on your Mac- Apple Support
2. Select: Disk Utility
3. Select: the Drive
4. Click: First Aid button

C. Remove Login Items:

While in Safe Mode, remove all of your Login Items, and then restart the Mac.  Go Here: Change Users & Groups Login Items Preferences on Mac - Apple Support

Run an EtreCheck report and post it in a new thread here with EtreCheck in the title and someone else here (not my area of knowledge) will likely respond. See: https://etrecheck.com/en/index.html