It's vehicle emissions. They all seem to use basically the same system, so it takes about the same amount of time no matter where you go. The only real variable is how busy they are. If it's 5-10 minutes per car, as I would estimate, you can get an idea how long you'll be waiting by how far back in line you are. I've had to wait here for half an hour or more, and sometimes as little as 5 minutes. This visit was 5 minutes.
The crew was pleasant, efficient, and got me out reasonably quickly. Apparently they now take credit (this is new to me), but probably a good idea to have cash in case there is a problem with the credit card.