Especially as restaurants have a limit on their capacity to prepare food. You can't just spin up another instance of a staffed kitchen. Do these mobile-food-ordering apps include any kind of backdown on order acceptance e.g. "Joe's Diner is too busy right now, do you want to wait or try someplace else?"

All you need for this is a dictionary of zip codes and a rating -- normal, high, very high. Given that ZIPs are 5 digits, that's 100,000 records max, just keep it in memory, you don't even need entries for the "normal" ZIPs. Even if you went street-level, I doubt you'd catalog more than a few hundred thousand streets whose income is significantly more than the surrounding area.

All of this ignores the fact that adjusting a restaurant's prices by the customer's expected ability to pay often leads to killing demand among your most frequent and desirable clientele, but that's a different story.