There is a little more to figuring out which is quicker.
This question all depends on the weight of the individual disk and drum AND the diameter of the disk and drum.
The drums are more compact being only 9" on the Mustang, while some disk brakes are 10-11" in diameter in the rear. SO which matters more? Being closer to the center of rotation...or being lighter in weight??
The math is too complicated for an answer on a friday evening