Talking whilst someone else is performing is bad manners. Any question, or answer, or comment, can wait until the end. Talking whilst someone is performing is you announcing to the world that you are more important than the performer, and more important than the entire audience. It is your comment that has to take priority over everyone else.

As for how short or unimportant the conversation was, who is to start drawing lines on how long a conversation has to be before it is acceptable or a nuisance? Are ten syllables OK, or does it have to be 20 to cross some boundary? How loud is acceptable - 20 decibels, 40 decibels, 80 decibels? The only boundary is being quiet, or not being quiet. Being polite, or not being polite. Being an self-centered arse, or not.

As for whispered asides, I suspect I have been guilty of some myself, in my time. But I'm not proud of it, nor expounding it up as a normal and proper way to behave.