Important eyewitnesses of the JFK assassination

One of the many eyewitnesses I found interesting was Gordon Arnold. Gordon was off duty from the military, so he decided to take his camera and film the president. When he took his camera to the grassy knoll, a man in the "secret service" told him he could not be their for "security reasons". After the shootings another secret service officer, or the same one, says to give him his camera tape. That seems extremely suspicious to me. However, when he went on the Dallas Morning News and was interviewed on what happened in Dealey Plaza, a man named Ralf Yarbourough, said he saw Gordon, ""Immediately on the firing of the first shot I saw the man you interviewed throw himself onto the ground. He was down within a second of the time the shot was fired" Am I the only one who finds that strange that within a second he was down. I understand he was in the military, but that is insanely percise reaction time.