You may think that but boarding durations have dropped in T5 precisely because of conformance checks landside. People arriving at the gate with the wrong removed staff from boarding the flight and meant that the flight docs had to be recreated. Much better to ensure that everything is in order before the customers have gone airside. There have been a few cases where customers have realised there is a problem (wrong passport) and because the check was landside and not at the gate at -12 they were able to get the right docs to the airport in time for the flight.