I have created a journal which is clearly too long. It is slow and often crashes.
Tony, there is no known limit to the size of journals. If iPhoto starts crashing, it is usually due to having imported media that cannot be processed (incompatible videos, corrupted photos) or simply, not enough free space on your device. Check the free memory on your iPad. If duplicating the Journal made the problem worse, it is a strong indication that you are running out of memory. Editing a journals needs ot of free space for formatting the photos.
I memory is no problem, try to remember, which photos you added to your journal most recently, and then delete them from your camera roll after saving your coamera roll to your computer.
Also, try the general trouble shooting measures described here: it may be a system problem and not an iPhoto problem.
iOS: Troubleshooting apps purchased from the App Store