I understand that. What I mean is the existing customers never get served after 8pm as the chefs never place any food in the windows for the waiters to serve to the customers. So, when the restaurant is closed at 9pm and all the existing customers get up to leave they have not had the food they ordered.
There is already two chefs and two cooking stations. The problem is not that they can't keep up. The problem is that they don't place food in the window to be taken to the customers after 8pm. It shouldn't take 3 hours for food to be made for one remaining couple of Sims, as is what happened when I left restaurant open until 11pm just to see if they would ever get the food they ordered.