I was at a watching party with a couple of IBMers who worked on Watson, and one thing they said is that it's not a question of speed, but of timing. Players time their pressing to an estimate of when Alex will finish the question, and Brad Rutter in particular has been clocked at under 2ms with shocking regularity. The advantages Watson has are consistency and the emotional perturbations in its opponents. You could see them getting frustrated, and that likely only served to harm their ability to hit that window between the end of the question and Watson's button press.