Is it really 15 minutes? What about all the time lost from flow/focus, the hour before the meeting where you are unable to start new work, the hour after where

Is it really 15 minutes? What about all the time lost from flow/focus, the hour before the meeting where you are unable to start new work, the hour after where you have to scramble to communicate/fix erroneous update info, etc. What if the scheduled time is right in the middle of your best focus time of the day? You've lost most of your day. every day.