At the speed of light you wouldn't see a difference between 101% or 99% speed I guess, especially in a local setting where the distance is tiny. That's why a super-highspeed camera is useful so you can observe.
Also, the light you can observe on the camera actually is not the light traveling between spot A->B. The light you observe has bounced of air or dust causing it to move into the direction of the camera. It shouldn't influence the speed measurement itself, but it shows there are multiple variables in such a 'simple' test to determine the speed of light.