I still believe that could indicate a code intense page. Both my PC's are using cookies but the one that has no antivirus/antispyware runs like a house a fire. When you're logged in more code is used by vBulletin to log stuff such as your ID, pages you visit/view control panel and the likes. Try this link and see if its slow loading.
http://vnboards.ign.com/board.asp?brd=5156