I've all but stopped using iPhoto to manage my images. It's so slow I can't even arrange folders in the sidebar. If an app dedicated to organization is unorganizable, well, I guess I don't really much see the point.
The thing is, there's no good reason for this slowness. I don't have that many photos, my hardware is reasonably good, and I've stayed up to date on the software side of things. So WTF?
Today I did some poking around and I was able to make iPhoto perform up to par again, after all these years, which is to say it's now working reasonably well. It's at least usable again.
Seems there was a bad cache file, of all things.
My bad cache file was located by:
1. Showing the contents of the iPhoto Library — located in ~/Pictures — by right-clicking and choosing "Show Package Contents..."
2. Deleting the folder called "Project Cache" which contained a long-forgotten iPhoto book project I was testing.
Since doing this iPhoto has been well behaved, but it's also good to know that there is also a cache store located here:
~/Library/Containers/com.apple.iPhoto
This is pretty great. Unfortunately, I've since worked around iPhoto's deficiencies by finding other tools for managing my images. But that's a story for another time.