Luckily for Barry, lightning doesn't travel at the speed of light, it travels at the speed of lightning, which is a mere 3,700 miles per second, which is as nothing compared to light, which travels at 186,000 miles per second.
His superfast reactions would mean he could go from "is that a gleam of light" to "Holy crap on a cracker, those two godless catamites are about to receive their justly earned divine retribution" at this sort of speed.