If they measured the time to go through the security check, the hard working last line of defence people may feel pressured to speed up and not ensure that the planes are kept free of people who will do us harm. AI'm quite happy for them to spend 30 minutes scanning my bag every time I go through, out of an abundance of caution -- surely 30 minutes of your time is worth it to stop the jihadiis from bringing nail cutters on board? Won't somebody PLEASE think of the children!