face recognition done in cloud?

What makes you think that the Face Recognition analysis hasn't already been done? I don't know for sure (hopefully an Adobe engineer could confirm), but I would think it's done in the cloud rather than in the local app. Bear in mind that all those "thousands and thousands" of pictures must be uploaded to the cloud (slowly), so I would have thought each image could be analysed as soon as it arrives in the cloud (there's no stop/start switch for face recognition in the cloud version of Lightroom).

One thing to understand about Lightroom's face recognition....it only shows faces to be named in the People view when it detects at least 5 instances of a face across the catalog (to try to prevent the view being swamped by unwanted faces from crowd scenes, etc.). Once a face has been named, however, it will always appear in the People view, even if there are less than 5 photos containing that face. To name a face that doesn't appear in the People view, because it has less than 5 instances, you would need to find an image containing that face, click on the Keywords tab (lower-right) and the people sections appears above the keywords in the right-hand panel. Any faces detected in that image will be shown and can be named at that point (if there are multiple faces on the image, hovering over the small icons in the list will show a rectangle over the corresponding face on the actual image, so that you can correctly match the name to the right person).