You bring up an interesting point and one I don't think I've heard in many "Live v. Frozen/Prekilled" arguments and that is the speed at which the prey item dies. Theoretically, the fastest way to dispatch should be blunt trauma to the head or cervical dislocation, but that's only if you do it swiftly and properly. Maybe constricting in 30 seconds is faster, but shouldn't we also consider the most painless and stressful for the prey item? After all, it's giving up it's life for your snake, it deserves respect. If we consider the most painless and calm for the mouse, the CO2 seems to be the best option.
I've been in a few debates. Most of the arguments center around damage to the snake. While yes, it is a concern, one does need to remember that snakes are designed to be as efficient as possible when it comes to killing. Blunt trauma can be messy (I've had mice bleed from the nose/ears because of it) and both can cause a huge amount of suffering if done improperly. To be honest, CO2 isn't all that painless and calm from what I've seen (one method I've seen is put the dry ice in a critter keeper, put it into an ice chest, add mice/rats, close the lid, and come back in five minutes). I've used vinegar and baking soda for CO2 since I can't find dry ice. Pretty much, the mice scramble around before going wobbly. They fall over, go bug eyed, and breath very labored until they die. This can take upwards of 5 minutes.
My snake on the other hand, would strike hard enough to cause the mouse's nose to bleed. She would constrict and the mouse would be dead in about 30 seconds. No matter how you look at it, the mouse is going to suffer. With larger prey items like adult rats, I'm sure you could put a CO2 powered BB gun to its head and pull the trigger (they have a decent amount of force behind them so theoretically, the BB should go straight through the head). This isn't an option for smaller prey.
It's something that requires a lot of thought, and in my opinion, something that boils down to preference. I know of people who have thousands of live feedings under their belt with many, many snakes and have had a small handful of instances. Others refuse to even own a snake that requires live.
Like I said though, I'm not going to condone anyone for how they choose to feed their animal (it is, after all, their animal). I can simply hope that they will do the same to me.