Depending on the type of person, they might respond better to numbers as well. Tell them: it takes XXX seconds for the website to load up on the old webpage, and YYY seconds for the new page. Then you can site research which shows that users have an attention span of XXX seconds, and will likely stop if it takes more than that. I think the number was about 10 seconds.
I can sympathize with your plight, though... I've been there...
