Hi, I have some people over here that have been experiencing what sounds
much like your issue. Several people here, slow to open shared calendars,
once open they can move back and forth between them okay - for a time.
After the stupid email team trying to pass it off as a 'network problem'
(idiots), I found this document. Check it out...

http://blogs.technet.com/gerod_seraf...20/405229.aspx

Hope this helps