We were just there last Wednesday night for dinner, and it took us an hour and 30 minutes to get seated, order, and eat. We had an ADR for 5:35, before the big dinner crowd came in. We had good service, very friendly and attentive. We were not in a rush, but we were keeping an eye on the time so that we could enjoy the Osbourne Lights.
As an aside, our food was disappointing. Stick to the basics: burgers, fries, milkshakes and you'll be okay. The atmosphere is really fun, though.
If you are concerned about making it to the Fantasmic show, maybe you should think about a counter service location instead. I think that the burgers would be just as good! JMHO
Good luck!