Interestingly, I always assumed that “fast food” meant the type of food; so a hamburger, even if not made from mass produced ingredients, would always be a “fast food”.
It’s a weird thing - people in the USA know what you mean by fast food and somehow McDonald’s under a heat lamp counts (rarer now but still found) but burgers at the gas station under a heat lamp don’t count.
Interestingly, I always assumed that “fast food” meant the type of food; so a hamburger, even if not made from mass produced ingredients, would always be a “fast food”.
[1]: https://en.m.wikipedia.org/wiki/Fast_food