I have actually found booking engines that deliver itineraries within 30 seconds all the time. Virgin Australia, Adria Air, Gol, Rex, Singapore Air and so on. They do it consistently. Why not Jetstar?
Do you think Jetstar is doing themselves a favour by having a system that might if your lucky deliver an itinerary in a presentable format for a client a day after it is booked?
Funny you've mentioned Virgin Australia.
AFAIK Virgin Australia and Jetstar are using same booking engine and are both fully managed solutions, i.e. no own hardware or software. There were huge changes in VA backend after infamous reservation system crash, but that still does not explain what you're experiencing.