I mean, what do you do in a drive through when you get to the window to pay and it's like 3x more expensive than you expected? Just drive away? I'm sure that's what people would argue, while completely ignoring that it's a massive social taboo and 99% of people would just pay because of social pressure.
Also, what happens if the place starts making the food after you order, but before you pay (so you can get your food more quickly and efficiently!). Have you committed theft, or defrauded the business by deciding not to pay? I mean probably not, but I'd hate to be the first person sued to test if it holds up in court.
There's a drive through problem, but it's not what you're describing. You know the price when you order and they're not going to tell you it's $10 when they give you your total then tell you $12 a minute later at the window.
The drive through problem is that many drive throughs aren't set up so people can just dip out of the line if they get to the order screen and see everything is way more expensive than they expected. So you'll have places like that where people have to wait in a line to not get food (slows down the line, and if there are enough cars in line people who do want food will go somewhere else).
Not that I disagree, but in my experience, the little displays that tell you what you're ordering (and how much it costs) are rarely used, and the worker on the other side often doesn't tell you how much it is until you get to the window to pay. So if you frequent a place often and already have a rough estimation of how much something is gonna cost (i.e. "I buy this meal at lunch every day" or whatever), and then you find out its twice as expensive, you run into the exact problem I've suggested.
You're definitely right about it slowing everything down though, and most drive-throughs get clogged very easily.