Adding keywords, Location Photos Big Sur

Do you have the iCloud Photo Library enabled?

Boot 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.

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

I am not using iCloud. After using the Photos app for several hours, the response got better - possibly the OS was continuing to perform the photo library update that was declared the first time I opened the Photos app after updating to Big Sur. If that's true I would suggest that it would have been better to either (a) perform the entire update before opening the library, or (b) perform the continuing update at a lower priority so as not to (significantly) interfere with the normal behavior of the Photos app.

Same problem. MacBook Pro 2 GHz 32 Gb, 41k photos in library on 1 Tb external USB3 drive. Setting a keyword, when it works, takes >10 seconds. Sometimes when I enter a keyword the system spins (with moving popup box), then keyword is lost. Also, with info window open, arrowing past multiple photos causes tens of seconds delay until the system 'catches up' to the current photo.