When you're not making a criticism, you can say: "You've got a bottomless pit" (you're making a bit of a joke about it) "You've got a large stomach (you may say 'belly' if it's to a child) "You've got a big appetite (this is being polite) When you're criticizing, you can say: "You're a pig" (this is very insulting) "You're a glutton" (less insulting than saying 'pig', but still critical)

There isn't really a word for this. It would be more natural to say "They can eat a lot really quickly." instead of using a single word to express this. You could call them a "glutton" or a "pig", but both of these words can be seen as insults depending on the context. These words also only express their ability to eat a lot of food and not how quickly they can eat the food. Edit: Actually, I thought a bit more on this and there is a word for this. "Speed Eater". A Speed Eater is someone who eats as a part of a competition, trying to eat as much food as fast as they can.

I know this is from a long time ago, but I would say "to gobble" means to eat a lot of food very quickly. You can also say "to gobble up" instead of just "to gobble". Someone who gobbles would be a gobbler. It's not used super often, but it is fun to say and a native English speaker would understand the word.