Children can go thru food 'jags', where they change their dietary preferences meal to meal, day to day, or every other week.  Hunger obviously can influence how quickly they eat, and hunger will be influenced by recent activities or inactivity, illness (do they have a 'cold' or is their stomach upset?).  They also can be distracted by anything in the surrounding environment -- other kids, pets, TV, insects flying around, etc.  Even something on their minds.  So all of these contribute to the speed (or slowness) that kids sit and eat their meals.

I have attached link for fussy eaters from the American Academy of Pediatrics.