for up to a minute

I'm not too well versed in animal physiology, but for humans it's closer to 3-4 minutes. And panicking doesn't influence the speed or terror much.

Drowning in water and suffocating with CO2 are the worst feelings of dread in existance. Animal or human, period.

And panicking won't speed it up, nor will doing medidation slow it down much since the body literally fights for its life and the adrenalin makes you very much awake and hyperalert during pretty much the entire process.

I think, it was by using nitrogen

You think right.

When panicking for lack of oxygen, the nervous system (mamallian and human alike) doesn't actually look for the lack of oxygen for some reason, but for the overabundance of certain "common displacers". Such as water and CO2. And that's pretty much the exhaustive list.

Nitrogen doesn't cause a panic response. Neither does CO (carbon monoxide).

You don't feel anything if you're suffocating in nitrogen or are poisoned by CO for a few minutes. Then you might feel a bit drowsy for some 10 seconds right before you lose consciousness.

Then you don't feel anything again. A few more minutes, and you're gone without even knowing.

Probably because Nitrogen is more expensive.

I believe, there’s a way to gas pigs that just makes them sleepy and then pass out. I think, it was by using nitrogen. But yeah, that’s not commonly done.

IIRC, it's literally any other gas except CO2 - the human body detects suffocation not by the lack of oxygen in blood, but the presence of carbonic acid (which is acidic). Since every other gas doesn't create carbonic acid (except maybe some gases that contain carbon?), they all silently/painlessly kill.

Nitrogen, argon, or hydrogen would all work and be fairly cheap. Hydrogen would be explosively flammable though, so you could combine two steps into one!