Magic keyboard

Try booting into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again. You might want to connect your keyboard to the iMac with it's USB charging cable so it will be recognized. The repeating should not make a difference in the initial boot. Once logged in use the mouse to navigate to the System preferences.

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

Otherwise you'll need to borrow any type of USB keyboard to log into your Mac and then start the troubleshooting process.

Check the System/Keyboard/Keyboard preference pane to see what the settings are for Delay Until Repeat. Change those to see if that will fix the problem.

I'm glad you got that epiphany and am up and running.