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.