I have three problems with this:
1. There's no explanation as to why this process is taking place.
2. There's no estimate on how long it will take.
3. Based on my estimate, it could take months.
There is a bit of an explanation in the Sierra preview: http://www.apple.com/macos/sierra/
Now Photos can turn them into unforgettable experiences called Memories. And powerful new technologies deliver improved face recognition, display all your photos on a world map, and even allow you to search your photos by the objects and scenery in them.
Face recognition needs a lot of processing power, and even more so the scan of your photos for content. Photos is analyzing your photos and tries to classify the objects in them, and assigns categories to the photos, based on the content. You can now search your library for photos of certain categories - try to search for dogs beach, birds, beer, mountains, birthday cake, combat ship, etc. Photos will know what is in your photos without you having to assign titles or keywords. I found the results impressive. There were a few false targets, for example a picture of a cat classified as a dog, or the pyramids of Gizeh classified as mountains, but most of the photos were labeled correctly.
This scan needs plenty of processing time. You cannot disable it, because all the new Sierra Photos features need the people and the categories, the people albums and the Memories.
For me, the most annoying thing about this huge amount of preprocessing us, that the results do not seem to sync between my Macs. It has to be done on all Macs independently.
The updated Photos Help says: https://help.apple.com/photos/mac/1.2/?lang=en#/phtf5e48489c
Note: People identified in the People album are not synced across devices.
So I have to name all people again , over and over on four Macs.