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.