Why is Photos.app taking 2 months to scan faces for recognition?
I solved the problem!  I hope this helps others. The following steps take a bit of time, but if you're like me, it was totally worth it.  Most of the work is done by the computer.  :-) 


Exported all my photos from my library as full-sized photos, choosing PNG formats, for optimal size without data loss.  This full export required some time to download missing photos from iCloud, but it reassured me that no photo information was lost.  It also detected all my "problem" photos that were corrupt or missing on the local hard drive.  And of course the exported photos serve as a crude but real backup.


Be sure to export the photos as full sized photos!  In other words, don't turn on "optimize storage" in this step!  

Created a brand new empty Photos Library, called "good library".  


Important:  This library MUST BE disconnected from iCloud at this point, i.e., is NOT the System Photos Library at this point.  Leave it that way for now, because if you connect this nice new library to iCloud at this point, you will end up with lots of duplicate photos on iCloud after step 3, which will then all show up on your phone and computer and screw everything up and you'll have to start over!  This is because the exported photos have different names than the old ones on iCloud and this confuses iCloud and it thinks they are new.  

Imported all my photos exported in step 1 into the new library.  Scanning resumed.  27,500 photos were scanned in four days at an average speed of 271 photos per hour.
Deleted all my photos from iCloud to avoid the duplicate photos problem I mentioned in step 2: 


Created a new empty Photos library on my Mac, called "iCloud download." 
I then turned on iCloud photos on my Mac and it's downloading all the photos to my Mac.  I have it set to "optimize storage" so it should be quicker than full resolution would be. 
In the iCloud download library Select-All delete all photos then empty trash.  DONE (some stragglers undeleted still)
I then confirm on iCloud (https://www.icloud.com/) and on my phone that all are gone.  Again, this is critical to avoid duplicates!  

After confirming that step 4 has deleted all my photos on iCloud, I switch to my new good library and make it the system library.  Photos got uploaded and problems were all gone.