I also pre-boil the potato slices a little bit. About 5 or 6 minutes.
This is a good idea, I don't know why I've never thought of it before. I often do my scalloped potatoes in the microwave (because who can wait 50-60 minutes for them to cook in the oven!?!) and finish them off in the oven. But next time I will par boil the potatoes first so they cook quicker.