It might slow them down more if you could hang the nets from above in such a way that there was nothing for them to brace the nets on and shove there noses between the holes.
Also, it might be prefereable for those feeding a small amount of a super high nutrition hay to go to a slightly lower nutrition hay (but still clean and good quality) so that you can feed more/longer without risking your horses health to laminitis, obesity, etc. Or supplement your high nutrition stuff with a filler hay, just to keep the digestive tract moving. (Again, still quality, clean hay, just low calories etc.) Obviously this won't work for everyone/every horse, but its a thought for those trying to figure out how to keep their horses bellies full longer.