My preferred and always attempted POI on game is a heart shot. I've flattened them on the spot with muzzleloaders, shotgun slugs, CF rifles and pistols. I've also had them with the heart blown completely in 1/2 run up to 100yds, with all the above mentioned firearms.
I do everything possible to avoid higher body shots through the lungs. I want two holes low in the body to promote, if needed, the best blood trail possible.
Place that shot correctly and you can kill them with a marble. I also believe it can depend on the alertness of the animal at the time. In general, an animal completely relaxed will drop much faster than an alert animal.
One of my earliest bucks shot with a recurve bow, was completely unaware of me or any other danger. He was feeding just outside the woods in a grass field. The arrow took him through the lungs and stuck out both is sides. He jumped straight up in the air and when he came back down, just looked around. Then he walked a short distance in circles and just laid down. Never knew he'd been shot.