I see Scott has addressed this and we'll be speeding up the start a bit.
I would comment in addition to what has already been said that what you see on film clips that show Mustang starts is not necessarily good starting procedure. I've seen enough "diversion from correct starting procedure" when it comes to starting a Mustang in my career to write a book.
Add to this ambient temperature and primer fuel in the lines that can as well affect the start as well as engaging the starter with the mags hot and you have just a sample of some of the several starting scenarios available to someone starting a Mustang
I've had Merlins crank for a considerable time before starting on cold days and had them kick over immediately as the mags went hot.
Starting a Merlin is never accomplished on a standard timeline. It's a variable for sure.