No, it wasn't quick. They used Carbon Monoxide and Hydrogen Cyonide.
The former was first used to exterminate handicapped Germans, whom were locked in trucks and then the pipe was attached in through a vent from the exhaust. That is obviously Carbon Monoxide, its effects on the human body's respiration system are well known.
Zyclon B (Hydrogen Cyonide) was the primary gas used, however. First used on Russian PoWs then later used on Jews to provide a quick, extremely painful method of wiping them out. Gasses weren't, as many seem to think, the primary method of execution in the Death Camps to any member of society Germany deemed unatural and unworthy of life. Machine gunning entire lines of people was the primary method, gasses were considered expensive whereas they could mass produce millions of MG42 rounds. Gasses weren't as widespread nor as favoured as many believe, but it was extremely effective.
But no, it was absolutely not meant to be humane. They also used Chlorine Gas and even combined Zyclon and Chlorine in tests. It would've caused immense amounts of pain, especially Chlorine Gas. It wasn't used as a humane option, the humane option was to kneel them down and put a round through their head, or line them up and attempt a shot at each head in the line. But Gasses were used as a painful, yet quick and easy to mass-kill, method of execution.
Accounts from several Death Camp Marshals reviewed that they joyfully used gasses because of the screams. I read that from an extract a while back, I'll try and find the source. The people were monsters, though there's a famous quote that the medical documentation the Nazis collected during tests advanced medical science by two decades. At the cost of nine million lives, one still doesn't deem it a worthy loss of life, which is why they rarely bang on about how much they learnt during.