Hi elizabethfromcambridge,
in my case, for "scanning Photos/Faces/People", it worked to activate the Energy Saver feature "Prevent your Mac from automatically sleeping ...". The screensaver was active, and the screen of my iMac was off over night.
What I did in detail:
- restart the Mac, make sure that no app is running
- check that the Energy Saver feature "Prevent your Mac from automatically sleeping ..." is on
- check with Activity Monitor that there are no "hanging" or stuck processes related to Photos.app. If there are processes that are not changing, quit them. Using the search string "photo" selects the relevant processes. The "circle with X" allows to Stop a selected process.
- open Photos.app - this should result in about 4 to 5 running processes detectable in Activity Monitor (filtered for "photo" in the Process Name).
- once these processes (including photoanalysisd and photolibraryd) have been started, quit Photos.app and leave the Mac alone over night.
For me, this has worked several times (e.g. after using the Repair function of Photos.app) to get about 40.000 photos/pictures scanned.