The number of photos I have in iPhoto is around 10,000. I like the new faces feature, but it just kept grinding my CPU for hours and hours and days and days. I stumbled on a way to rebuild the iPhoto library, but it’s really hidden and I’ve never run across this type of a setup from Apple Software.
Essentially you need to hold down Option + Command while starting iPhoto and keep them held down until you get a window asking you what you want to rebuild. Interesting. Maybe I’ll report if solves the Faces issue with the CPU.
Did this help you out? It took me a few days to piece together all this information together, I hope this saves you some time (who knows, maybe the future me will be thankful I wrote this down). Let me know your thoughts. firstname.lastname@example.org