When I go out to eat with Dine Out, I tend to just order one drink and one food item such as a main course. That tends to fill my sims up, so I don't tend to order more but in theory, you can. I guess it is set up that way because if you want to have a long lunch or drawn out date, you can keep ordering courses. In practice, I tend to just order a meal.
Sometimes, I have a group of sims where one is less hungry than the others, they might just get a drink or an appetizer or dessert. That helps to keep them at the table. The annoyance that I have is the recent tendency to wander off or to start dancing in the restaurant. I have to repeatedly cancel out the dancing and to repeatedly direct the group to sit together which is annoying.
I guess that dining out goes slower or faster depending on how much my sims socialize, including before they request a table or order. Every so often, you get a restaurant with poor service and then it is really slow. They'll drop dishes and bring poor quality food. Obviously, that's just part of the pack. I tend to say that most of my sims won't go back to a place where they experienced bad service at least for a while (or until I forget about it).