For the most part, the food franchise machine is a thing of the past, however the cooks of Medieval America certainly take a few pointers from the low frills, high-calorie, and most importantly, portable foodstuffs that industrial America was infamous for. Boiled eggs , bread, and donuts are the most common foods for those on the go, but in a larger towns and cities where the populace is big and the citizenry does not have access to hearths, there are few popular street foods inspired by fast food cuisine
Hamburger sandwiches aren't unheard of, but for the quickest turnover, many ovens will churn out the Big Mac, which is something like a cross between a calzone and a pie, with ground meat, onions, pickles, curds, and maybe potatoes wrapped up in a bread casing.
Pizza is very convenient because it serves as their own plate (but it's very unappetizing when cold). Not all of them use tomato sauce, at least not all year. In the colder months when the plant stores are low, it's not uncommon to use butter or certain fish oils in its stead. Obviously, one can't order Domino's on the phone anymore, but some ruling houses may have a recurring pizza delivery on retainer.
The combination of a bread roll and a sausage made from dubious meat parts is as popular as ever, though lack of refrigeration has moved hot dog vendors away from the Oscar Meyer wiener, and more to the more easily preserved smoked and spiced sausages
Popcorn is not too difficult to make by hand, and is especially convenient as kernels can be preserved almost indefinitely if they're kept dry enough. This is of no small importance in a world of no refrigeration and possible crop failure. As a result, it's most commonly eaten over the winter, and is actually considered something of a Christmas staple.
Soda is not nearly as widespread as it was, but having a coke is the drink of choice for the wealthy in Mexico and the American southwest. It's largely because it has access to the ingredients from South America, cane sugar, and ice from the mountains And yes, "ingredients from South America" means cocaine