Firing squad would be faster. But then they would not suffer as much.
Guillotine.
Quick, certain, supposedly painless, and because you're not putting chemicals or current through them, you have transplantable organs.
My preferred method is simple and cheap.
And artsy if I may say so.
Load up a crew of them once a month from whatever State into a C17 then shove em out over water from ~15k ft. (keeps em a tad cold but conscious all the way down.)