Seems to me that there are a number of things that cause the backups at security, and would respectfully disagree with FWAAA by observing that when someone is emptying his life's possessions, taking off shoes, and doing the pre-screen ritual right at the scanner, they can't not be slowing the line down at least some - unless the scanner is already backed up. But I would agree that these folks aren't the only slowdown. Depending on the checkpoint and the screeners, sometimes it seems to take an Act of God to clear individual items past the Xray'ers, and when this happens, it can really put the brakes on things. I don't want to second guess the screeners, but it does (empirically) seem to go slower than it needs to sometimes.
As for me, I prep before I get to the checkpoint, so I'm ready to rock 'n roll when I get there ... 2 laptops out and ready, shoes off, nothing in pockets, no sharp objects, and anything questionable from inside my carryon out and passed thru the Xray in one of those cute doggie dishes.
One thing that would really help is to have a few "reassembly" tables available just past the secure area to allow pax to repack their stuff. Some airports, like my home base here in Richmond, VA, have that, but many don't - which can cause clogs when people bunch up in cramped quarters to stow their things. Now that they're not doing the boarding checks anymore, they ought to have plenty of spare tables for this...
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/globaldude