Facial recognition not working after Sequoia update

As a data point, I tried putting 1500 pictures with the same person in them into a new empty Library. It took about a day to recognize all the same faces as were recognized in the original Library. 60000 might take longer. This was in Sequoia, and on my internal drive.

• Is your external drive an SSD formatted in APSF, or what?
• How much free storage do you have on your Mac? At System Settings>General>Storage mine says 1.4TB of 2TB used. What does yours say? Does it indicate that you have at least 15% free?
• You don't say-- are you using iCloud Photos or Optimize storage?
• Have you used any 3rd party "cleaner" apps or virus eaters or VPNs?

To narrow things down, try these steps:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder. 
• Rebuilding the Library was a good test.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

Also run the test library on your boot drive a well as the external SSD. That can tell us if it's the external nature of the drive is part of the problem or not or if it's a system wide problem.