Talk to some of the LHR screeners when they're not too busy. They realise and privately admit it's a farce, and in LHR's case, compounded by management's total reluctance to have enough staff on duty to handle the traffic. Unlike Nice the other week, where the queue started getting longer so they kept opening security gates. Or PHX the other Sunday where the queue moved so fast you hadn't a chance to put your carry bag down to grab your documents!
Of course, the SLF don't help. It's been made public enough that you must remove shoes, so women turn up with thigh length tight boots that almost need machinery to extract them from.....and cans of booze, etc.