what +Raymond Dickey said.
i have seen young (~14 weeks post-egg) Opsanus tau - oyster toadfish - cannibalize one another. they will snap at most anything smaller than them, including fellow hatchlings. at least once, a largish toadlet grabbed another's tail, and inched (millimetered?) slowly up the other fish's body, shaking and struggling, gills pumping. three hours later the unfortunate sibling was near-swallowed bodily up to the gill arches, which were still pumping (though much more slowly). after that i wanted to see no more.
and i've seen much the same with garter snakes (Thamnophis sirtalis) and toads, even once where the (smaller) snake got the (relatively larger) toad by the hind feet instead of the head... the whole process took the better part of an hour.
given these are vertebrates, and given a cold-blooded metabolism with low oxygen requirements, deaths in these kinds of cases - which surely happen every day - are surely agonizingly slow, suffocating nightmares for the animal being consumed.