I would definitely go down to smaller hole nets. 2-innch holes don’t slow them down at all IME.
Both my air fern Fjords get all their hay in 1 inch nets. It frustrated them a little bit at first; just for a few days though. They figured it out pretty quickly and now just take it in stride. Even at that, if I feed according to optimal weight (1.5 to 2%) they do not have hay in front of them at all times. One of my boys is just a much faster eater than my other one; just their own habits.
At best the 1-inch nets maybe slowed their eating time by 50% (what used to take an hour now takes 1 1/2 hours or so). It’s a trade off keeping them from being too overweight. Maybe try more frequent feedings of a single flake at a time with a shorter time (hour or so) of no hay between feedings.
I have not tried the 1/2-inch holes, but am tempted. They are harder to find though and I do need to replace nets at least 2 to 3 times a year. I have tried double bagging the 1-inch nets when they get worn, but that does seem to frustrated the boys more and they get rougher/yank harder on the nets, so not sure that an even smaller hole would work well for them.
I use the $16 nets available in a number of places on line. Made by Partade, I believe. Wish there was something reasonably priced that lasted longer. Friends who have spent much more, still complain about replacing more often than they like too, so I guess it’s just the cost of keeping fat ponies healthy/IR at bay.
One other recommendation my vet had was a low dose of Thyro-L to keep weight down. You may want to ask your vet about testing your horse’s thyroid level. Both mine did, at different times fall into the borderline low level. They are looking good and keeping at an appropriate weight between the Thyro-L, hay nets, grazing muzzles, and fairly regular exercise.