Here's how I solved this problem: I had the same issues as everyone else here and tried all of the same remedies reboots, killing processes, repairing library, leaving Photos open but minimized, closing it completely, etc), with very little luck. Here is how I finally got the people scanning to finish scanning 32k photos...
I killed all of the "Photo" processes in the Activities Monitor >> CPU Panel. Then I rebooted my MBP and I didn't open ANY programs at all (except the Activity Monitor) after about a minute, the "photoanalysisd" process started running and eating up 100%+ of the CPU. I then simply turned the brightness all the way down on my monitor (F1 key) to avoid sleeping the screen / hard drive and the "photoanalysisd" process ran for 13 hours 47 min of CPU time.
The key for me was to not open any programs (including Photos)... I could get the processes to start running after a reboot, but as soon as I open Chrome, the process would stop.
All 32k photos are now scanned for faces. Hope this helps!