I think people confuse airport congestion with security holdups. Carry-on x-rays have been mandated for a very long time (long before 9/11). And people always had to walk one-at-a-time through the metal detectors. What's happened over the last 25 years or so is a massive increase in air travel in the U.S., without a corresponding increase in airport capacity.

Edit: I think there's also a lot of inefficiency. So often I see a huge line, half the equipment sitting idle, and half the TSA agents standing around talking to each other. I don't remember that sort of thing in the before times. Maybe because the security people were employed by private companies motivated to get their money's worth.