Way back when, I wrote about the addition of premium passenger security lines at Dulles and I later wrote about the change in the security gates which seemed to seriously degrade the speed at which the premium passenger line moved.
Around the time I noticed the movement of the security lines to the western end of the airport, I started bearing right when coming through the tunnel from Daily Garage 1 if I had already checked in online (bearing left would bring you to the United checkin area). When I came up to the security area, there was another Premium Passenger line, this one you enter on the western end of the airport (opposite end from United). This Premium Passenger line was always much shorter than I remembered, but I didn't put A and B together till my current trip.
I had forgotten to bring along my boarding pass that I had gotten during the online checkin and so I had to checking with United. After checkin, I walked down to premium passenger line near United (well, not "near" but closest). There was a sizable line there, so I decided to walk around to the other line and voila, the line was much, much shorter (as in 5 people).
Moral of the story: walk to the western premium passengersecurity line -- especially when the airport is crowded.