There can be several reasons:

• Your site isn't visited very often, so the asp.net has to startup on each visit
• Your skin is poorly designed
• Your database could be slow
• Compression or cache settings
• Your host could have you in an application pool with many other users
• Hardware setup/problems, network, etc...

Do a google search on these forums, and you'll find some reasons. Also try from different locations to ensure it's not your rig.  You can also look at the performance section under the host menu. Also take a look at page blaster from snapsis. www.snapsis.com