A dog who guards its food and/or eats everything fast and all at once may have had to fight for a place to nurse or eat while younger. Once way to make you dog feel more comfortable is to get down near the dog while he/she is eating and play with the food. Some dogs will get aggressive and may offer to bite, so be careful. With enough practice, your dog should begin to relax instead of feeling like food is going to be scarce or whatever.
Slowly decrease the amount of meals he/she is eating. If he/she is able to eat his/her meals whenever he/she wants, then the dog/bitch will realise their is no need to rush the eating of food. And may often wait several hours or minutes before starting and/or finishing the meal.