When you go to the Exchange 2007 OWA page either from inside or out side of the network, the page loads fast then after you authentic you wait about 30 seconds to see the Left side nav bar then wait another 30 seconds for your messages to come up. After that it is fast in what ever you do. If you log out and then back in it is fast, your immediately back into the inbox with your messages.
This is for all users and the server is not behind any ISA box just behind a sonicwall router will 443 pushed thru. I just put in a full GeoTrust cert but it was doing the same thing with the self made one that exchange sets up.