It does take considerable space. I upgraded from iPhoto '09 to '11 and the simple upgrade with faces added 7GB of useless garbage to my machine and the dumb program slows my iMac to a crawl. Stupid. I will not be upgrading anything in the future if this is how Apple now operates. Seems like a MS way of doing things. How often do I want to search for all the pictures of a certain person anyways?
I have iphoto 9. The way I solved this was to select a 'face' in the faces panel, and press cmd-backspace. It got rid of the faces, and when I removed them all: Voila! no more useless crunching of my CPU.
Maybe so, but it certainly does not thrash the disk, and there is a very noticeable improvement in speed. Hence, it may be 'scanning for faces' but it does it faster.