As with many eating disorder behaviors, eating slowly is often a good and healthy idea. European cultures take longer to eat their meals, and the experience is healthier physically and emotionally.
If you are consistently not finished with your food when others are done, it may mean that you are looking for a reason not to eat as much. There's nothing that says you have to "clean your plate," but if you're eating slowly just to avoid eating a normal amount, it could mean that you are dealing with an eating disorder.
