The index process is reasonably fast. Each face needs to be recognized five times for the system to be able to prompt you for a name for that specific individual. I can pretty easily infer what they were trying to accomplish (avoid prompting you for transient people who are not in your circle of family/friends; e.g. some random individual on the street). Personally, I think this a very stupid system; especially since the system "never" goes back and refinds someone new your circle on previous pictures.
Overall, the face recognition aspect of Lr sucks compared to probably every competitor out there. It also, fails on the manual work around aspect also. I have yet to find any actual redeeming "feature" about it.
In terms of the sharing. I cannot answer, I do not use that feature.