I've got a user on Outlook 2007 connecting to Exchange 2003. When she opens an email with a return receipt, it takes about 30-60 seconds. I've found one reference to this problem via search, but no solutions.
The machine is a 2.6Ghz P4 w/ 512MB DDR266. One thing of note is that the user's mailbox size is very large (>3GB). Regular messages open fine.