I guess someone's got to be daft enough to ask, so it might aswell be me: if you're using a second zone as audio only, why is a delay a problem?
In theory at least, I would imagine there would be LESS of a delay on a second zone as there's less processing involved. As the AVR doesn't measure any second rooms it's presumably playing it in Pure Direct?
Are you sure that the second zone is the one delaying? Try playing the main AND the second zones in Pure Direct and see whether they time any better.