I wouldn’t bring it up. He’s probably heard it before and doesn’t need his girl’s uncle pointing it out.
I drink a lot but how many fifths are you knocking back each day?
I’m a fast eater, Ms. P is about average. Our younger son has always made fun of how she pushes food around on the plate, but she never struck me as slow. We had a chicken farm until I was around 14, and it wasn’t rare for people to show up wanting eggs during meal time. That made a fast eater out of me.
Why not… just ask him if he prefers to have the cake now or wait until after everyone else is done eating? I feel like you’re imposing a lot of expectations onto this that no one else in the situation shares. This does not seem like a difficult dilemma to resolve.
Also, OP, maybe if you ate faster you’d no longer be starving
He may well not be lusting after the delicious sweets at all, though. I mean, I’m sure your cake is lovely, but if it was me, my personal relationship to cake is “oh yeah, cake. Mildly pleasant if it happens to be there. No problem waiting”. Especially if he’s a food-as-fuel person.
Basically this is just another “Eh - don’t sweat it” post 
Add me to the chorus of “say nothing”. The main likely effect of saying anything will be that your niece and her boyfriend will giggle about her weird aunt once they’re alone. You’re responsible for providing an environment in which he can be comfortable, and you’re feeling like maybe you’re not doing that by leaving him with an empty plate for so long; but if you criticize, however obliquely, his eating choices, you’ll definitely not be providing him with a comfortable environment.
My ex BIL was a super fast eater. I’m talking three minute meals. He chalked it up to his time in the Greek Navy where you would have 90 seconds to eat on many days.
Another ‘not a problem unless you make it one’ person here; a friend I often go out to eat with is like this- I’m normally considered a fast eater, but I swear the food on his plate just vanishes. In this case, he’s a chef, so food is to be snarfed as rapidly as possibly on breaks, but it in no way indicates he doesn’t taste or appreciate the food, in fact he’s got much higher standards for where to eat than I do, and will usually come up with a full critique of the meal afterwards, despite the fact that it apparently touched his tastebuds for about a picosecond.
I can joke about it with my friend, but I wouldn’t suggest you try that with this guy, at least not yet.
I’m a fast eater. Between Army and a law enforcement careers, I eat like someone may take my food away at any moment. I’m polite and all, but I do eat very quickly, and always the first one done at the table. I actually find it easier to engage in the dinner conversation after I’m done eating.
Perhaps semi-related: At any sort of social gathering where there is food being set out to self serve, be it a backyard barbecue, or a catered class reunion buffet, I am that guy not afraid to be first through the line letting everyone else know it’s OK to eat now. 
You keep doing the Lord’s work, you hear me? We need more folks with that kind of courage.
I guess I’ll be the guy to trot out the old prayer:
Father, Son, and the Holy Ghost
Whoever eats the fastest gets the most
mmm
Heh. At my brother in laws birthday party, I was sitting at a table with my Wife (BIL’s sister) my nephew (his son) and a few others. All the food got set out buffet style, but no other dinners at other tables touched it. Well, says our table, lets dig in. And we did.
Well, I guess everyone was waiting for a mass blessing of the food.
Everyone else got up and gathered around the food. I thought “Hey, perfect picture opportunity!” And started snapping away. Didn’t realize they where doing some sort of prayer thing.
We were the secular table I guess.
This!
I’d much rather eat for 5 minutes non-stop then talk for 20 than eat & yak simultaneously for 25.
Why? Because my way the entire meal is hot. The other way 80% of the meal is eaten cold. Screw that.
Now if the event is cold snacks, or we’re working over the cheese & fruit course or the chocolate course after dinner, then yakking mixes fine w eating. But no talking while the main course is cooling.
Have you considered hiding caltrops in his food?
LOL @kaylasdad99 ! As a slow eater married to a fast eater I can see that this might be tempting…
I’m a fast eater. I tried to slow down in slow eating company but it feels awkward so I just eat how I do. Im last in line to eat, but will eat every single thing on my plate and won’t ussually accept dessert or seconds. I’ll engage in conversation as I’m engaged, otherwise I’ll just listen. I dont mind either way. If you need help cleaning up I got that covered or anything else.
To me it’s necessity based(I’m solving a hunger problem). Yeah, the food was awesome. I did indeed taste it, it was wonderful, thank you for asking.
Eh, sometimes it is just family culture. I have a slight tendency to talk over people when I’m relaxed but also excited/wound up over a conversation. I’m a lot better about that bad habit these days, but it’s still something I have to watch out for. Where did I pick that kinda rude conversational tendency? Well, I was reminded one year visiting a slew of my father’s relatives for Thanksgiving.
Everybody gathered around happily and loudly talking over everybody else for a couple of hours before the meal in a total state of ear-splitting conversational chaos. It was almost like a competition 
.
Then the food came out and there was maybe twenty minutes of dead silence as everyone got their eat on. Many of this side of my family were pretty hefty and we love our food.
Then…meal is done. And hours of loud, animated conversation starts back up almost simultaneously for everyone.
So meal times were for conversation with that crew, but the actual eating was break time from all the loud talking. Possibly started as a necessity to help rest strained vocal cords for the next round.
I know this makes me weird, but I’ve never understood the appeal of hot food. A warm beverage, sure. But food needs to settle a little so I can eat it in comfort.
Yes, I’m the person who is routinely burnt by pizza. No, I don’t enjoy it, neither when it happens, nor the next day when I have a little flap of dead skin in my mouth. Steak is lovely when it’s slightly warm. So are most veggies. So long as my food isn’t cold enough for the fat to have congealed, it’s plenty warm.
But piping hot food can cool down. It’s sort of like how I’d rather have cold weather than warm because you can bundle up against the cold. I prefer my food somewhere on the spectrum of “warm” but I’d prefer that when people prepare me food they intend to make it either hot or piping hot so that it’s guaranteed to not be cold.
So long as it’s neither piping hot nor icy cold, I’m probably okay with it.