Imac running slow

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

That being said uninstall Norton Security according to the developer's instructions. You can check to see if you've removed all of the supporting files by downloading and running the shareware akpp Find Any File to search for any files with the application's or the developer's name in the file name.  For the Norton software you'd do the following search(es): 

1 - Name contains norton

2 - Name contains symantec

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

If you can't delete any found files because they are running (in use) boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

You could benefit from adding an additional 8 GB of RAM (two 4 GB modules from Crucial.com or MacSales.com).

the only possible culprit i saw was symantec. you should completely uninstall it following the developer's instructions. so-called "security" apps are not needed on macOS and can cause performance issues by conflicting with apple's built in security.

please read Effective defenses against malware and other threats - Apple Community. it's a long article, but it's worth reading every word.

you're welcome.