One tip that seems to make a HUGE difference in how well LR identifies faces and how quickly you can tag people.
Instead of staying in the master view where multiple people are tagged at once, drop into a single person view by double clicking on the name/photo of a person listed in the "Named People" list. That will take you to a sub-view showing the Confirmed photos of that person, and below will be the Similar photos of the same person. Lightroom seems to have a much higher degree of accuracy finding similar photos as compared to attaching names to a wide swath of people. I've found that I can quickly tag hundreds of images using this method. I'll bounce from person to person, and the more I tag the better LR seems to get at finding similar images.