While it is rude to honk immediately when the light turns green, it is also true that taking too long to accelerate when the light changes severely reduces traffic flow and causes congestion. And this is commonly caused by distracted drivers. You are supposed to release your brake and begin moving at roughly the same time as the car in front of you, i.e. when the car in front of you turns off their brake lights or when the traffic signal turns green if you're in the front.

Edit: "Too long" realistically means anything more than half a second. If everyone's reaction time was one second it would reduce the number of cars per green light probably by half. That doesn't mean it's polite to honk after half a second but if your reaction time is that bad you should not be driving.