Like the .270 short - but why that's considered slow when an honest 2900fps from a 16" barrel is possible with hand loads, I don't know.
Really, the comparisons should all be made vs. 5.56, which is exactly what the originators did. The 5.56 does just fine in combat to incapacitate - stop - another soldier fighting. That's an entirely different concept from the humane kill of a game animal and preventing it's loss from poor shot location, the real issue.
Larger calibers have no guarantee they will stop and put down an animal - it's difficult to measure or even quantify "knock down" power in any sort of meaningful way. What we do know is too little, and the animal won't quickly bleed to death, too much, and the shooter does, can, and will anticipate the recoil in a negative manner.
Anyone who's shot magnums, or got hit with a scope bell knows exactly what I'm talking about. Shooting "main battle rifle" calibers can even do it. That negative perception of what a "big" gun does is a documented and proven distractor that causes the shooter to second guess his shots and use shooting methods that impede accurate shots at moving targets.
Hunting is a lot about shots at targets that don't simply pose or graze (at least in my experience with whitetail deer.) Having hunted for decades being the only semi auto gun seen in a conservation area or during chance encounters in the field, I know for a fact that there are a lot of manual action gunners out there spraying the woods trying to knock down a running deer. Having attempted to use a bolt or lever gun, I know it impedes a quick and accurate second shot following up the first.
I'd rather use a self - loading action in a caliber that can humanely put down the animal if the first shot is off, and use that gun more confidently to get a good shot knowing I won't inadvertently eat a scope bell or get whacked by excessive recoil awkwardly tracking a deer in a tree stand. It's not easy to maintain your balance on a 18" square platform made from a few sticks of welded tubing 12' up a tree traversing a 150 degree arc.
If it makes the difference in placing another round to humanely put down the animal, why do so many misunderstand and get a gun that's too big and causes MORE problem? Bluntly, they don't think, they let testosterone make the decision, the same as when they chose a homecoming queen or buy a truck instead of a car. It's image, societal pecking order, or just plain lust, but it's not common sense.
God forbid we buy something adequate, and learn to use it well. The typical situation is to buy something in excess, and then never really learn to or need to use it.
I hunted with a .308 HK91, and Aimpoint, it worked ok. Then I tried a .30-06 with scope, it wasn't ok, moved to a .30-30 lever, nice and light, but still didn't load itself. Back to a semi auto in the AR, I've used it 22 years, know it, know the 6.8 is every bit as good as the .30-30, America's #1 deer caliber, just trimmed to fit the AR action. A little faster with a much flatter trajectory, more range, even better optics, and NO PUNISHMENT. It rewards the shooter with so little recoil you concentrate ON THE SHOT, not on getting bruised.
That's why the AR in 6.8 IS the huge success it is less than ten years from it's introduction, rocky a road as it's been. Really nothing more than most other cartridges, if anything, less so. The .30x.223 has been a very minor player since the '70s, and most PPC cartridges are doomed to stay on the target range, guilty by association with precision shooters - the truly glorified ones spoken in reverent tones are always military. It cuts both ways, not getting adopted doesn't help them either.
If what 5.56 offers isn't enough, you move up, 6.8SPC offers it in the best package the least expensively in a commercial caliber for the hunter.