They're arguing the wrong issue. The proof of the conspiracy is the
timing of the shots, not the length of time to fire all three shots. Arlen Spector created the Single Bullet theory to account for Kennedy and Connally being hit almost simultaneously. The Zapruder film show JFK reacting to being hit, and shortly after, Connally being hit. If they were hit by two separate shots, but within one second of each other, then you have evidence of a conspiracy, since Oswald (or whoever was firing) would not have time to reload, aim, and shoot. Therefor, the commission had to reason that they were hit by the same bullet, and created the Single Bullett Theory.
Any reasonable person looking at the Zapruder film can see that they were hit by separate shots. And that is where the most obvious evidence of conspiracy exists.
I’m a reasonable person. From the Zapruder film shows them getting hit at nearly the same time (since they are emerging from behind the road sign, exact point of impact isn’t possible to determine) by the same bullet.
Therefore your hypothesis holds no water.