It was 50 seconds from bite to cough. I assumed it was closer to 1:20. I think what can explain why nobody else at the front tables ate was due to pausing to watch Joff harass Tyrion. He snatched the plate from Margaery as she tried to feed him the second bite. I think I was wrong, this does all make sense for letting Joffrey be the only death. Most weddings would involve Margaery taking a bite before feeding Joff the second, but it can easily be reasoned that this wouldn't be the case since Joffrey is Joffrey