Some individuals are inherently slow eaters.  There is some suggestion that those who eat slower are less overweight, partly because they give their body a chance to recognize when it has enough nutrients, enough food to take care of its energy, protein and other needs.  It is very difficult and probably unnecessary to change your child's eating habits.  If your child starts missing events (e.g., sports or play activities) due to this eating behavior, the behavior will likely resolve when there is incentive to eat faster.

However some children may be eating slowly because of physical reasons;  if your child appears uncomfortable while eating, or complains of painful swallowing, food getting stuck, heartburn, or regurgitates, then you should discuss this further with your child's pediatrician.