Gypsy, the Equine Encyclopedia, has something to say about slow feeders. Like all Shetlands, she will do anything for food.
“Slow Feeder” is a general term for a haybag or feeder that has mesh or bars to prevent horses from eating too much too quickly. Slow feeders also help keep hay clean and off the ground. This, however, does have its disadvantages for if a horse has a sensitivity to dust or has heaves, it is best to feed him on the ground.
Ponies can outwit slow feeders and actually eat twice as fast when using the proper technique.