I had this exact same experience with LHR (London Heathrow). I was never even trying to leave the airport, just make a connecting flight.
Not only did they decide that having a single security lane open was sufficient, but they also (effectively) only let one person unload their bag into the trays at a time, for a max bus throughput of 1 person at a time. If you left anything in it - and I mean anything[0] - they pulled your bag and tray to the side, and you had to wait for one of two agents to inspect it. The wait time for that was about 10-15 minutes. At no point did anybody have any sense of urgency whatsoever with their work, despite all of us being there to make connecting flights. They shuffled back and forth between the body scanner, X-ray machine, and the break area, chatting with each other. I was seventh in line, and in all, the entire process took literally 30 minutes (I timed it).
I was floored, because I've traveled around the world and never seen anything this bad. Even Heathrow security has never been that bad for me when taking off from London. There was an agent from our departing flight there to escort us, because we were holding the whole flight up. I asked her if this was typical or not, and she just rolled her eyes and laughed, "yeah, this is how it usually is".
[0] One guy next to me had his back selected because he had left his eyeglasses inside his bag, in a plastic (not metal) case.
A couple of years ago I had to take a connected flight where one of the stops was in USA. I was never going to leave the airport, yet I had to form in a line while immigration was doing the routine bottleneck check on dozens of travelers, while my flight time was getting dangerously close. We just had to suck it up, any complaining just brought additional delays. The check was fairly automated, I guess they just needed more agents, or less gratuitous revisions.