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.
The time limit is 10 seconds for "keeping the user's attention," 1.0 second for "the user's flow of thought to stay uninterrupted," and 0.1 second for the "user [to] feel that the system is reacting instantaneously." -- http://www.useit.com/papers/responsetime.html
Then show them some fast loading sites, their compeditors. Then ask them "I am an aol user with an attention span of 10 seconds, where would I go?"
The irony is that they are AOL users who said that they don't like accessing their site from their dial-up modem at home -- they only want access it from the DSL at work.
