I have a late 2015 iMac with 8GB memory. Apple Support has looked at it and can’t figure out what causes it to run so slow. When they screen share, they are surprised how slow it is. Still have a 2007 iMac and it runs great. As I mentioned, it is likely caused by something outside of the OS. It wasn’t always like this. For example, we have wiped it clean and restored from Time Machine with no improvement. They looked through Utilities and didn’t see anything that was causing the problem.
If you want to start a new thread, I'd be happy to help you troubleshoot this. I've never seen a slowdown problem that I've been unable to diagnose.
The first thing that I would do would be to have a look at this Web site and see if anything there helps:
Macintosh Slowdown Solutions
Macintosh OS X Slowdown Solutions
You are a previous Windows user. I bet you downloaded anti-virus software first thing. If you did, it's probably strangling your Mac. Completely uninstall it (this may require an uninstaller program from the app's developer) and see if things are better.
None of the above helped? Try this: Create a new user account on the computer, with no third party software installed,
On your Mac, choose Apple menu --> System Preferences, then click Users & Groups.
Click the lock icon at the bottom of the pane, enter an administrator name and password, then click Unlock.
Click the Add button at the bottom of the list on the left.
Click the New Account pop-up menu.
Enter a name and password for the user.
Click Create User.
now boot from it (choose Log Out in Apple menu, then log back in via the new account). Are things better? If they aren't, the problem is almost certainly a hardware problem rather than a software problem.
If the problem continues to exist when logged into a fresh new user account, then I'd download:
DriveDX - free demo
DriveDx - the most advanced drive health diagnostics and monitoring utility
and see what it says about the health of your hard drive.