Inside the iPhoto Library Package, there are four SQLite databases (face_blob.db, face.db, iPhotoAux.db and iPhotoMain.db). I found that by vacuuming all four of these databases, I restored iPhoto '09's performance back to something on par with its predecessor.
Open Terminal and cd into your iPhoto Library (by default, cd "~/Pictures/iPhoto Library"). Once there, execute the following command:
for dbase in *.db; do sqlite3 $dbase "vacuum;"; done
[robg adds: Although I wasn't experiencing any slowdowns, I tested this command just to make sure nothing broke ... and nothing broke.]

