I would understand if it were many for an individual. However this happens if there are 2 photos for a person or 20. I double checked and my grandson (in the attached photo on the OP) is only tagged in 7 photos.
One thing I have found is that if you let it 'sit and spin' for a minute or two and then cancel, often it will come right up. Not always but a lot. Sometimes that just results in waiting again.
So it seems like it is going through all photos for the site rather than just that individuals - at least at first. Or it is building some kind of cache and gets hung up and then when you cancel and restart the process it is done.
Am I the only one seeing this? Or perhaps the only one reporting it...