I've always had better success with just loading skinny horses up with enough hay that they never run out. We tried hay nets in an attempt to keep hay from blowing away in the pasture and the skinny ones deemed it as too much work and barely ate a thing. We've never tried to put weight on an underfed normal keeper though.... just REALLY hard keepers whose owners had been struggling the entire time they owned them to keep them at a decent weight.
I always had free choice hay for my horses, long before the slowfeeder idea was born. And I was very reluctant at first. But I had to do something for blimp, no way around. I noticed they favored the nets over hay on the ground even( for the transitioning phase I had both, nets and on the ground). It takes about a week for them to learn that there's always something, no need to gobble it up as fast as possible.
I, of course, spoil my horses....I have oat hay, alfalfa and orchard/ timothy, a net of each, spread out, so they must move, and they do! A nibble here, then off to the next, even leaving the alfalfa to get a mouthful of oathay.
I still give a little oathay inside the sheds, but they pick out the kernels, the rest remains for bedding.
I weighed what I put in and raked together and weighed again what was left, for a month, before I started with the nets. Average 28 lbs per horse, and about 10lbs wasted. Then I did the same with nets...15 lbs eaten, 2-3lbs waste. And still hay in the nets.