It's not like we didn't have airport security lines before the TSA. And with the new millimeter wave machines I'm not sure if they're appreciably slower than the old metal detector setup. The bottleneck was always putting your luggage through the X-ray, and we've had that for as long as I can remember.
I don't have the data to back it up, but from what I have personally seen, the bottleneck is now predominantly the body scanner, mostly because many airports are running a setup where they have two x-ray baggage detector lines feeding into one body scanner.
The bottleneck is definitely the xray scanners. I've never seen the line be held up by people waiting for millimeter wave machine. Also, if that line gets even a little long, the TSA agent will wave people through the metal detector real quick.
That said, things like taking your shoes off, taking your laptop out, removing belts, etc. really slow down the xray machine line.
To be fair my observations are solely based off n~= 20 flights through generally smaller airports, but I have seen extreme reluctance on the part of TSA staff to pass people through the metal detectors, especially during times like holidays. I do agree that the security theater surrounding the x-ray bag scanners is also a significant issue and might be the primary bottleneck at other airports or at different times.
When I was flying in continental USA on a weekly basis in 1998-99, security lines were seldom a problem. On more than one occasion, I parked my car in the DIA short-term lot (client was paying) 20 minutes before flight time. Gate agents would wag their fingers, and I'd be breathing hard when I got aboard, but I never missed a plane because of it. Also when I forgot to stash my pocketknife in my carry-on bag I just plunked it down in the change bucket with my keys and change. It really was easier and more enjoyable to fly back then...
If you think it was a joy to fly then, you should have seen it before the security measures adopted around the time of the 1990-1991 Gulf War and associated perception that Iraq would target US flights for terrorist attacks.
I think the key bottleneck increaser was the requirement to remove shoes and to remove more things (electronics & liquids) from luggage when being sent through the x-ray.
