Let me give you an example, you run at 12 miles per hour in one direction, and a car is going in the opposite direction. The actual speed of the car is 45 mph, but because you are also moving, its speed from your perspective changes, making its speed 57mph. This means if you would have to react to a 57 mph object as opposed to a 45 mph one. The only thing this doesn't work with is light, and that's because lightspeed is a constant.
Boros was lunging at Saitama when he threw the punch, meaning he would have to react to something much faster than just his movement or Saitama's punch.