Security queues vary enormously between two mornings at the same airport. The screening process barely changes, so the variation comes from how many people arrive at once and how prepared they are.
Throughput is set by the slowest step
A screening lane moves at the pace of divesting, scanning and re-packing, and the scanner itself is rarely the constraint.
A passenger who empties pockets, removes electronics and separates liquids before reaching the tray clears in a fraction of the time of one who discovers each requirement in turn.
Because trays feed a single belt, one unprepared passenger holds up everyone behind them, which is why throughput depends more on the mix of travellers than on the equipment.
Arrivals are lumpy by design
Flights are scheduled in waves to suit connections and airline rotations, so passengers do not arrive at a steady rate.
A bank of early morning departures concentrates thousands of people into a short window, and the queue builds faster than any number of lanes can drain it.
Airports respond by rostering staff against the flight schedule rather than the clock, opening lanes for the peaks and closing them in the troughs.
Queue design changes perceived time
A single queue feeding many lanes is fairer and faster on average than separate queues, because one slow lane no longer traps a particular group.
Visible movement matters too. A queue that snakes and advances continuously feels shorter than a static one of the same length, and complaints track the feeling rather than the minutes.
Some airports add lanes with multiple loading positions so that several passengers prepare simultaneously and the belt is never waiting on one person.
Passengers are separated by risk and by speed
Trusted traveller programmes screen enrolled passengers through a lighter process after checks done in advance.
The gain is partly procedural and partly compositional: the fast queue also contains frequent flyers who know the routine, so both queues speed up when they are separated.
Paid priority lanes work on the same principle without the background check, selling a shorter wait rather than a different standard of screening.
Rules change and the queue absorbs it
Screening requirements are set by regulators and revised as threats and technology change, and equipment is replaced gradually across a network of airports.
During a transition, requirements can differ between terminals, which produces passengers following the rules they met at their departure airport.
That mismatch, more than any single rule, is what creates the unpredictable delays travellers notice, and it resolves only once a rollout is complete.