Something I wrote from a previous thread. The one piece of info that wasn't in this comment is that Watson can NOT anticipate when Trebek is about to finish the question. Watson must wait until it gets a signal that the buzzer is now available, and only then can it begin the process to physically depress the button.

Remember that Watson also has to depress a physical button (the same buzzer everyone else uses). The eye to finger path for humans is about 200ms. It probably takes about 100ms for Watson to physically press the button. So Watson is about 100ms faster. But that also gives humans about a 100ms window in which to beat Watson. This means that you need to start your press 100-200ms before Trebek finishes his last word.

That's pretty good sized window for most people given you are reading the question along with Trebek. If the person who turns the light on is very consistent, I think a human who is good at this could consistently beat Watson.