Let’s take your car.
Depends on many factors.
If you had come to my drive through during lunch rush, a 20 car line means that you are through it in around 5 minutes, as I would be moving a car every 15 seconds.
It depends. Last weekend my daughter was in a dance performance and afterward she asked to stop at a Sonic restaurant. We went to the drive through and there were 6 cars ahead of us. But it went pretty quickly. They even had a young woman walk out to our car to take our payment before we got to the window. It was really efficient, we must not have been in line for more than 10 minutes.
If it’s 15 seconds, I’ll never get inside a fast food restaurant again.
When fully staffed with a good crew, that was about as fast as we could move cars. We could actually move the food a bit faster, but the cars couldn’t keep up.
We weren’t leaving the inside service wanting either.
When I worked, I also smoked tobacco. Drive-throughs allowed me an extra 5 minutes of smoking time, so I always used them. About a year before I retired, the drive-throughs all put up no smoking signs by the speaker, signs by the lane and by the window(s). I was always a considerate smoker, so I stopped using the drive-throughs. Of course, as long I was parking in the back of a parking lot so I could smoke on the walk inside, I stopped getting fast food and started getting something healthier.
Sure you can. People do it all the time.
Riiiiiight… I can just imagine lugging all that on an airplane or train… 
I still don’t understand getting in a line of cars at Sonic. If there are one or two cars I might use the drive-thru, but anything over that, park in a stall, place your order, and let the nice person bring it to you (and consider tipping them a dollar).
Or use the app, which usually removes almost all of the wait time.
That may be true where you live- but it isn’t everywhere. Although I live in NYC, drive-thrus exist outside of Manhattan (I 've never seen one in Manhattan) and they are often near homes and pedestrians.
That takes longer, and it’s really difficult to get in and out of that particular location unless you’re in the drive through. It’s not set up well. I’ve had bad experiences getting Sonic in a stall there. It’s the poster child of prioritizing the drive through.
I haven’t done the app at Sonic but every other fast food place I’ve used an app for, I arrive, let them know I’m there, then sit forever for my food. They don’t work. They’re great in theory but fast food restaurants give low priority to apps. It’s a big disappointment. I’m not banking on the idea that “it’ll be different this time”.
I would like to thank the Original Poster, because looking up “drive-thru” led me to an interesting Wikipedia page:
I have never been to a Chick-Fil-A at lunchtime that didn’t have a line snaked around it like it was the last helicopter out of Saigon. But it moves fast.
They’re a model of efficiency: employees walk the line with tablets to get orders and take electronic payment, they tell you which car you’re following at the merge, and your food is invariably ready as soon as you reach the window. It boggles my mind how much drive-through business they must turnover in a day.