I'm curious about the size of your bales as well.
I feed 800 lb. bales, and have two horses and a 10 hand pony.
Just putting the bale out on ground, the bale would last 7 days, using a round bale feeder, not a slow feeder, takes it to 10 - 12 days by eliminating waste. This math works for me; I assume the horses eat about 35 pounds per day, or 2/3 of a standard square, and the pony eats 10 - 12, also 80 pounds per day, 800 pound bale lasts 8 days. It may also be worth mentioning that my guys eat no grain or concentrates, have their heads in the bale non stop, and are in good flesh bordering on fat.
So I seriously question both the bale weight and the quality of the hay. If they're both eating 100 lbs + a piece, they should be waddling!
All thatn said, a slow feeder will probably pay for itself in a season and is a great idea.