It's already been discussed how to handle a nursing baby who gets distracted instead of eating: What works to get a distracted nurser to eat during the day?.
But what strategies can I use to help a child of preschool or elementary school age who exibits the same pattern? Eats a bit and then gets distracted by... well, pretty much anything. Talk to whoever is around them (on any topic), think about stuff, play with utensils, look at the stuff in the room, sing/hum.
This seems to be an issue of distraction: they exhibit the same even when eating their favorite foods, but sometimes (rarely) given sufficient motivation they can breathe through exactly the same meal in 15-20 minutes instead of the usual 60. And the meal goes a lot faster if there's an adult in the room CONSTANTLY reminding them to concentrate on food (like, every minute).