From one of the linked articles by a reporter in the room:

The gas appeared to start flowing at approximately 7:58 p.m. Smith visibly shook and writhed against the gurney for around two minutes. His arms thrashed against the restraints.

He breathed heavily, slightly gasping, for approximately seven more minutes. At one point, his wife cried out.

At 8:01 p.m., a correctional officer in the execution chamber leaned over Smith and examined his face, before stepping away and walking back to his post. Smith appeared to stop breathing at 8:08 p.m.

I've experienced hypoxia and found it very comfortable, if I had to choose a way to go there would be it, but that was at a high altitude and my brain had no idea what was happening (later edit: done properly with no carbon dioxide buildup, the brain should be unaware with nitrogen too - I suspect their untested mask was shit). This sounds barbaric.