We don't store anything you type into forms, and don't store any cookies any longer than the browsing session is alive (currently expires after 5min of inactivity). But you're right of course, as a user you have no way of verifying this. The main scenario I was envisioning here is things like news sites, which often go overboard with complex scripts, CSS, images/videos/flash, ads, etc, to the point where a simple news article is difficult or impossible to read - especially on an older device or on a slow 3G connection.