Q: How long "face scan" 100,000 photo in Photos App?

Is it really take such long time for scanning?

Yes. I haven't attempted to perform anything more than a cursory estimate, but I believe it took about an hour to finish a library of about 1000 photos.

When it's finished... it's pretty amazing, but I'm not a professional photographer. Photos is not just a simple organizational tool any more. For someone with 100,000 photographs you probably already have an organizational plan, so perhaps you won't find those automatic features as useful as I do.

I understand. Please read macOS Sierra: If Time Machine remains in the “preparing” stage. With 100,000 photographs, your Mac is going to take a very long time.

When it finishes (and it will) its performance will return to normal.

It's going to be a long wait. You're the first person I know of with that many photographs.

Hello, it took approx. 16 hours for my 15.000 pictures. So you now, about 1000 per hour

However, after that, the faces were recognised quite reliable.

FWIW, I would imagine things would go somewhat faster if you temporarily disable Time Machine. Less disk thrashing.

The face detection has been considerably improved in Photos. And the  "photoanalysisd" is not only scanning for faces, it is also scanning for categories, so it can classify a photo as a birthday cake, a dog, a glass of beer, or similar. The object recognition needs a lot of processing.

My library has 45000 photos and the initial scan on my MacBook Pro's took roughly 3 days. The full sized originals in my iCloud Photo library are stored locally, so no download from iCloud was necessary.  On my iPhone, with the originals stored in iCloud, the san took mor than a week.