Thanks.
How busy do the queues usually get for the quick service places?
The mobile ordering is quicker than just waiting for someone to take your order once seated?
If you are on the quick service plan it is all fast food, you order at the counter (like McDonalds) or use the app and then collect from the counter.
The only places that are different are
Be Our Guest (breakfast and lunch only, dinner is table service) you need a reservation and you order on the website or at a kiosk in the restaurant and then food is "magically" brought to your table.
ABC Commissary in Hollywood Studios, dinner only you order at the counter and food is brought to your table
D-Lux Burgers in Disney Springs, you order at the counter and food is brought to your table.
There is also a prix-fix menu at Pizzafari at Animal Kingdom at dinner that is ordered at the counter and brought to the table, but regular fast food service is also available.
Queues in general will be bad at lunch (12-2) but most move quickly (although I have waited 15 minutes just to order before), or you could use mobile ordering to order on the app and just collect from the dedicated counter.
At busy times you may then have difficulty finding a table once you have your tray of food.
Also having said it is fast food, it is not all burgers and fries, there are a lot of great options like Flame Tree BBQ at Animal Kingdom or Tangerine Café in the Morocco area of Epcot