Session's is slowing the page ??

Hello, I have very wierd problem : I use session to manage users and I have this structure of the page : ----- include("db.php"); include("header.php"); bla bla include("footer.php"); ----- in the db.php I have : ---- session_start(); #some mysql connect stuff ---- I use session with the $_SESSION array (cheking if registered[isset($_S...)] and registering...) and all work fine.... but sometimes the page loading is very very very slow (and its a really really fast host) and when I removing session_start(); from the top of the page, the page load normaly. the PHP version is 4.1.1, someone have an idea ? thanks in advance, Dima.