It does SEEM like most of the people were not reacting to 3loud rifle shots in the timely response that we as observers 60 years later, are speculating they theoretically should have.
One would think the combat veteran Brehm at least would have recognized the very FIRST loud noise as a rifle shot and have reacted quicker in some way like diving to the ground.
It could be that the 1st loud shot everyone heard was not that different from some previous motorcycle backfires so they may have already been slightly desensitized to a loud noise by that.
However if there were 2 more shots that were equally spaced apart about 3 secs each and the total time was 8-9 secs, then is it not more probable that the Newmans and the 3 men on the GK steps and those women right near the JFK limo should be more animated than they appear to be?
So this suggests the 2/3rd ear witness descriptions of 2 shots very close together was an accurate evaluation and that 3 shots were fired more likely in about just 4.8 secs of time.
If so, then this might explain why the reactions which seem delayed are because of the ”shock effect” of an extremely horrifying stimulus being completed in such a short span of time.
This is why imo, 3 shots over 4.8 secs (substantiated by closest earwitness Harold Normans spacing demonstration) with the last 2 shots only about 1 sec apart leads to a higher probability that a semi auto rifle was the murder weapon rather than a bolt action MC rifle which with rate if fire at best only 2.5 secs per shot on average.
Note: there was ONE old guy on the CBS time trial shooting experiment that they CLAIM shot 3 shots with the MC rifle in only 5.1 secs and scored 3 hits on the moving target body ( but missed the head entirely)