Finder stopped responding on my iMac after deleting a large group of files

What files did you delete and where were they located?

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 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

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Boot into the Recovery volume (boot with the Command + R keys held down  - Intel Macs) or  (How to Boot an M1 Mac into Recovery Mode), select Disk Utility and run First Aid on all volumes possible. If you get an error message rerun immediately and repeat until error message goes away. I wouldn't go past 5 runs. Then reboot normally and see if there's any improvement.

At this point I'd boot again into Recovery mode and reinstall the system. It won't overwrite the user data, only replace the system and with it the Finder.

It's time to take the iMac into the nearest Apple Genius Bar or Apple Authorized Service Provider for a diagnostic check and to fix the problem.  

When you say a “large” group, do you mean the files were large, or that there were a large number of files, or both?

if you are syncing Desktop and Documents in iCloud, you may have just made a massive change to a folder that needs to sync with the cloud in near real-time.