Heat is lost yes, but cooks using our drums have been clocked in at getting back up to temp within 3 or 4 minutes after closing the lid, sometimes less.  That is the design, and operator experience at work.  Pro Tip:  You have to be careful how long you leave the lid open while checking on and/or handling the meat being smoked in any way.  The internal temperature of the cook chamber can easily get to a hotter temperature past the max you’ve set once the lid is closed again.  Why is that, you ask?  Because, you’re essentially stoking the fire by having the lid open.  So, you have to be a little careful not to leave it open for too long.  Another Pro Tip:  This one’s for when needing to have the lid open for a little longer than normal.  Let’s say you’re moving or removing a brisket for example.  Closing the dampers completely while you have that lid open will help your fire not overshoot the desired set temperature you want the cook chamber to get back up to after closing the lid.  Once lid is closed then open up the dampers again.  Read more about what causes temperature loss inside an upright smoker