A well-trained younger person can sling more lead than us older guys. However, I have found that with a moderate degree of practice, the average shooter can manage one shot per half second. When you think about it, that's three shots in one second. A shot at the beginning and end of the second and one in the middle.
I once witnessed Tom Campbell of team Smith and Wesson, empty a nine millimeter pistol so quickly that with my back initially turned to him, I thought someone had fired a burst from a sub-machine gun. He's a world-class shooter but it takes enormous practice to maintain that rate of fire.
Int'l Assoc. of Law Enforcement Firearms Instructors