Once your Photos syncs and finishes downloading from iCloud, in order for the "People" to scan you need to SHUT DOWN Photos but leave your computer running. According to the software, "Photos will continue scanning your remaining photos when you're not using the app and your Mac is connected to power." Once this process starts, click on the "People" page and it will tell you how many photos have been scanned and how many the computer has left to scan. For me, this has been an agonizingly slow process. I started timing it when I had 45,422 photos scanned (out of around a total of 90,000 photos). 1 day later 53,596 photos had been scanned. After 2 days I had 65,176 scanned, after 3 days I had 78,972 and after 4 days I had 80,709 scanned with just over 10,000 left. I'm guessing that the more photos the computer has to scan, the longer it takes to search through all those pictures.
Another "nasty" thing I noticed is that all of the photos that I had previously edited (before installing Catalina) have mysteriously LOST their editing! The edited thumbnail photos show the symbol that they have been edited (three small white horizontal sliders in the upper right corner), as does the bigger photo when you double click on it prior to editing. HOWEVER, when you click on EDIT, all your previous adjustments and cropping are GONE and what you see is your original photo as it came from the camera. If you then click DONE with editing the image is now your original from the camera photo, but still has the "edited" symbol. Double click on that to reduce it to a thumbnail and you see the EDITED photo again! Sadly, if you EXPORT the larger photo when it looks okay (before you click on EDIT), or if you export the thumbnail that also looks okay, what exports is the original from the camera photo (not the edited version). I'm hoping that eventually the actual data containing the "edits" will also be downloaded -- but what are the odds of that happening?
Any pictures loaded and edited AFTER I got Photos reloaded from iCloud -- the editing works as usual.
Do others have this editing issue ... or is it just me??