Apple - MacOS: Annoying lag when typing

The first thing I would try is starting your iMac in Safe Mode as follows:

1. Fully shut down your iMac
2. Restart your iMac
3. Immediately press the Shift key and keep it down
4. Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
5. Take a note of what happens (i.e. do you still notice the lag)
6. Exit Safe Mode by restarting your iMac as normal
7. Test again to see if the lag still persists

Once you've tried the above let me know if the lag was still noticeable in Safe Mode and also when booted in normally again.