iPhoto will rebuild the thumbnail cache, or rather ask you if you want it to be done, every time it feels the need. Sometimes this might not happen for a while and with a huge library your scrolling can become crawling if things go bad.
I have a library of some 1800 images and after doing clean OS X (Tiger) and iLife 06 install I have imported my images back but scrolling was, to say it mildly, a joke. After two days of using it iPhoto didn’t ask me to rebuild the thumbnails cache so I had to ask for it.
After exploring the menus and not finding anything there I have actually tried a few key-combinations while starting iPhoto up and there it was – hold Command and Option down while starting iPhoto. The result is the following prompt:
Now select what you want iPhoto to do and click Rebuild.
Update 06 Oct 2007 - The original article was written with iPhoto 06 in mind, but the same works with iPhoto 08. The only difference is that iPhoto 08 has an additional function to examine and repair file permissions for iPhoto gallery.
