In minor competition the starter shot the gun,the sound travels until the sprinters.Since(at 15°C) usually the speed of sound is about 340 m/s,and the starter is usually 15-20m from the sprinters,it take 0.06" before them can begon to react.
In major competition,behind the block we have a small megaphone connected with the gun.does this connection provide an immediate responde?If yes, we can say that in minor races the athletes are slower depedning from the starte position? (0.03" for 10m distance).