As someone who works in the food industry (tho not fast food) I’m not sure how you’d practically implement your suggestion.
In your example I’m sure you could have the register tally orders taken/cashed out, but there are a lot of positions where such tally would not be possible. It also would then become very difficult to get people to work for all but peak periods (as their rate of pay would effectively go down when not processing customers). It also doesn’t take into account the fact that a lot of customers are in fact the slowest, least prepared and socially aware people in the world at the moment they are interacting with customer service people. It also doesn’t account for the fact that people want niceties when interacting with a customer service person like small talk, and that would go out the window, and additionally that companies themselves want food employees to spend time trying to “upsell” people on sides, premium condiments, fancy drinks, desserts on their order which is directly opposed to processing customers as fast as humanly possible.
Additionally, I’m not sure companies would be keen on your suggestion as they would end up spending way more on employee wages even if a customer service employee was given a few cents per guest.