I've done site work in an abattoir and saw a skilled worker humanely 'bolt' 10 cattle in just over 30 seconds. This included moving from one stall to the next. Yes the guy was on bonus but each kill was verified as 'clean' by an inspector. You cannot get more humane than that.
And for the record, after witnessing this I came out of that shed and emptied my stomach contents on the floor outside.
Just a side note, these bolts don't kill...so imagine a paralysing bolt of electricity that stays in you body and brain for around 10-15 seconds but still allows the brain to register firing from nerve endings.
Ever had an electric shock anyone? What about one for 10-15 seconds without being able to move?