For average restaurant service, I will leave a minimum of 15 percent for a tip.
First, if I give my complete order when a server first comes to the table. That is also how I receive my meal. The drinks, appetizers, salads and meal all come at the same time. I came to a sit-down restaurant to enjoy a meal and socialize with a lady friend.
If I wanted fast food, I would have gone to a drive-through window. I will purposely order my drinks first. When a server returns with my appetizers, I will order my meal.
I am not really concerned that I am holding up a table. After waiting in line to get seated, I feel that the table is mine. If I receive good service, I will compensate accordingly, probably with a tip in the 20 to 25 percent range. I also am not concerned if the restaurant is busy. I am also very courteous to my server and I expect the same from my server, no matter how busy his or her day has been.