I have (had?) the identical problem, with just slightly different details (I'm on Lion, not Mountain Lion, and I haven't tried repairing anything yet in iPhoto). iPhoto used to start up in a reasonable amount of time, but it seems since updated to 9.4.2, the startup time is incredibly long (I'll guess 5-10x longer). Admittedly, I have one of the largest iPhoto libraries I've heard of, but I'm just really trying to see why the update changed startup time so much.
Note that I have 500GB of free space on my boot drive, and that creating a new iPhoto library results in a speedy startup.
An oddity I just now see after creating a new library is, when I go back to select my old library at iPhoto launch, there are two libraries listed, with identical names, and identical paths ("iPhoto Library" - /Users/UserName/Pictures/iPhoto Library). Looking in the finder, I only see one Library with that name. After playing around with opening either one, now the first one says "(default)" after it every time.
Note that I just added "(had?)" to the beginning of my post, as it seems like my library has started opening in the normal amount of time since I played around with the libraries (creating a new one, etc.). So maybe just creating a new library and then going back to the default library with help thatgurlwho too?
I have this excruciatingly slow start-up with iPhoto as well. To add insult to injury, often after iPhoto is finally running, it totally slows down everything to the point that I have to re-start.
On an archived posting with this problem Apple fixed this with a firmware update to the NVIDIA GeForce 7300 GT. I have an NVIDEA GeForce 9400m, but cannot find anywhere what the current firmware version is supposed to be...