Apple Photos face recognition: cloud or local? I am using Photos on:


iPad Air 2013 A1474 / iOS 12.4.6
MacBook Air 2013 / OSx Catalina 10.15.4
iPhone 7 Plus 2013 A1784 / iOs 13.3.1
MacBook Pro A1989 / OSx Catalina 10.15.4


All devices are set to synchronise iCloud Photos "Optimise Mac Storage", with the exception of the MacBook Pro, set to "Download originals to this Mac"

On the MacBook Air I have imported thousands photos from older folders on disk. It took a few days to upload to iCloud and sync on other devices.

Once done, for some unknown reasons,


on MacBook Pro, people and pictures are set and done. No curation
on iPhone all pictures available, people search doesn't work (empty) results when selecting people
on iPad it took days to build thumbnails, people search limited to few results
on MacBook Air, people recognition is not working yet as it is 4 days the computer is "Curating" (tried to reboot twice, still curating): "5.114 photos scanned photos will continue scanning your remaining 24306 photo when you're not using the app and your Mac is connected to power"


I wonder why such a different behavior... 
Is face recognition performed in the cloud or locally on each device?
Is face recognition performed in the cloud or locally on each device?


According to Apple: https://support.apple.com/en-us/HT207368

“When you search your photos, all of the face recognition and scene and object detection are done completely on your device”

Now, about your issue, here are some things you should know and can try that might answer your question. 


in order for your devices to build up the curation of faces it needs to be plugged in (according to the initial pop-up you receive after purchasing a new device and opening the photos app)
As you take photos the curation process will always be running, since you have a huge database of images, allow your device to sit with the photos app front and center. 
As far as I am aware, Apple does not automatically add people to the People album (probably to save processing power). You must find a photo of the person you would like to see filtered from your library and enter a name. This will add them to the People album and begin sorting all images of that person immediately from all photos. 


I just tried this with my iPad and iPhone, and both devices have empty People albums until I add my name to my face manually and then all photos where I appear are sorted. 

Edit: Faces that appear constantly throughout your library might appear in the People album on their own, but you should still add whomever you desire manually for best results.