The clearest reaction [to the first, missing-everything, shot] is JBC's head turn [back to his right] at Z-164. A shot at theoretical Z-124 would have been heard by him at theoretical Z-126. That's 38 frames, or just over two seconds.
Between Z-133 and Z-143, JFK is looking at the spectators to his right (and either waving to them or brushing his hair back),
as is JBC, who is just looking at them.In Z-143, JFK starts turning his head quickly towards Jackie and then turns it quickly back to his right, towards the spectators.
At Z-150, JBC quickly turns his head to his left about 45 degrees and is "captured" looking almost straight ahead in the Z-161 Robert Croft photo.
In Z-164, JBC starts turning his head quickly back to his right about 120 degrees to try to catch a glimpse of JFK over his right shoulder, but he can't "see" (i.e., recognize) him because by Z-180 JFK has turned his head far to his right and has raised his hand and forearm to wave to someone, thereby partially obscuring JBC's (peripheral) view of his face.
Point being: JBC consciously reacted to Oswald's "Z-124" shot about 1.31 seconds after hearing it, not "just over two seconds" after hearing it.