Hmm... I guess officially I'd have to say during the Coruscant speeder chase sequence in AOTC. It was the first time in my life I was downright BORED while watching an action scene in a Star Wars movie. I remember watching it in disbelief, looking at my watch thinking "What's wrong here? This is Star Wars, I shouldn't be so bored!" Then the love scenes & asteroid chase sealed the verdict.

Though prior to that, repeat viewings of TPM had given rise to stronger & stronger worries that this was going to blow.