Four seconds is short too.
I understand what you are saying. But in no way (in this context) is four seconds “a very, very short amount of time”.
Yet in that “very, very brief span of time” he had enough time to recognize it as a rifle shot, conclude that an assassination was unfolding, turn around in his jump seat to attempt to see JFK, and then decide to turn back the other way. Nellie even said he uttered “oh, no, no” before the second shot.
But we don’t have to interpret JBC’s statement. Many others did that. The interval between the last two was about half that of the interval between 1 and 2. With Oswald shooting, that requires about 4 seconds between 1 and 2.
Besides, if you think there was an early first shot miss, you don’t accept that his “very, very brief span of time” means what you are suggesting.