Strange - but I think you've identified the cause of the problem in your own analysis.
I did my dev. testing (and periodic checking) using the most popular browsers - IE (5,6,7), FF, Safari & Opera and the site performs similarly with them all. The only differences between then would be caused by how one browser interpreted the CSS compared to the other which would show up as a variation in the layout.
Given that page load times were pretty consistent across all of them, I'm afraid I can't make any meaningful suggestions for solving your Avant problem.
Mike
