We are creating the forum selection list in the moment the visitor clicks on it and (optionally) are also checking the permissions whether the visitor is allowed to post in the forum or not. This takes some time (up to 2 seconds), especially at larger forums. There are similar add-ons which create the selection list at each view of forum home, no matter if the visitor will click at the button or not. It is our philosophy to not harm the performance of forums with our add-ons. This is why we don't give all visitors the malus of creating the list. You also have the option to deactivate the permission check to speed things up, if someone clicks the "Create Thread" button or navigation link and you've a XenForo web site with many forums.