A good number of Drupal sites use front-end Varnish proxy to ensure pages load faster. The most commonly used Varnish configuration caches pages for anonymous users and lets requests with cookies pass through to the backend.
At times, a contributed module updates the $_SESSION variable, setting up the session cookie even for anonymous users. This increases page load times. You have probably encountered this problem several times. To fix this problem, you need to know which module is updating the $_SESSION variable. Sometimes this is not immediately clear. This article will detect the module that is updating $_SESSION.