Horses as you know are trickle feeders they are not set up to deal with the sudden arrival of highly palatable food. If you really need to feed concentrate it is best fed, not in a bucket but spread in a line on the floor. This way the horse can forfil his need to move while eating. Watch a horse grazing in the field and you will see he will step+eat+step+eat. This should solve the problem as well as adlib forage at ground level, not from a haynet that will also cause frustration.

My horse does the same and I find it helps to add hot water rather than cold. I don't know why but it stops her choking. Dry chaff is the worst as she swallows without chewing and gets it stuck in her throat. Her dentist also recommended placing a mineral or salt lick in the centre of the bowl so she has to eat around it and get minerals at the same time.

My horse used to bolt incase other horses came to take her feed, she had been very hungry in a "past life" and I used to worry sbout colic. I use Dodson & Horrel Equine Sensitive which is made from extruded nuggets and it has to be chewed, otherwise horses can't eat it. It definitely slows mine down and now that she is in the habit of chewing and taking her time, the idea of bolting her food has gone away. Also I should make sure your horse always has ad lib hay so they don;t get very hungry and need to rush their feed. Hope this helps.