Split second as in actually significantly under a second? I've never been able to get mine to take a picture without lighting up for very nearly a full second.
Consider that the optical nerve is shut off when eyes move from one target to another. What human "sees" during that time is a retained image, so he is effectively blind. If you make something appear on screen away from his focal point it might distract user enough to be blinded while his focus moves across the screen, so the green dot won't be seen.
It really was significantly under a second. This was a much older model of MBP, though. A comment elsewhere in this thread suggests that maybe Apple switched from simply wiring the LED in with the camera to using a microcontroller for this very reason -- so the light would stay on for a minimum amount of time.
