The results depend entirely on the concentration. At a concentration of 8%, CO2 acts as a pain killer. As the concentration gets higher, the anesthetic effect acts more swiftly. However, high concentrations can cause distress.
However, tests on rats showed that a gradually increasing concentration of 33% caused no notable stress, and cardiac arrest within 5 minutes.
It takes some practice to use a CO2 chamber properly. Namely because CO2 meters are really REALLY expensive, so "doing it by hand" is often the method of choice. However, if you are killing them in 30 seconds, the concentration is WAY too high.
In my experience, if they are going to sleep within 2-3 minutes you are doing it right.
It's impossible, so that's a fairly reasonable stanceAnd I can't see CO2 staying in high concentrations in a feeder for so long that it is harmful to the snake..