Yeah, come to think of it, using the infamous psychological term here... correlation does not imply causation. I had a hypothesis that lower Se traits could make one less aware of their external environment, thus leading to slower reaction time because said visual input was not encoded because not enough attention was made to the stimulus, leading to a delayed reaction. But from what those of you sensors have said, that those with really excellent use of Se that experience the whole world around them, even they can be caught up in what they are sensing and then have slower reaction time too.
So, I think that it would be more valid to say that poor reaction time, on a physiological level, is due to poor motor-coordination skills... either from lack of use, lack of development, or eye-hand coordination problems.