Theoretically, if they just have a slower reaction time than you, you are actually slowing them down as they now have to look around to make sure you aren't honking because of potential danger, which is the actual usage purpose of a car horn.

Edit: Additionally, this creates a broader societal danger as the "go when honked at!" mentality is entirely opposite of the true purpose of a horn.

Example: Person in front is about to go, you notice that someone is running the red light perpendicular, you honk to signal the danger, now instead of recognizing the danger, they go anyway, thinking you were impatiently honking, leading directly to an accident.