"How painful CAN it be" is the question OP asked, which I would interpret as "how bad can it possibly be".
SN can be very bad. I'm not a medical doctor, but as far as I understand, what happens is that your blood physically can't transport oxygen anymore, and you suffocate to death. No matter how much you breathe, it's impossible for you to bring that oxygen around. Way different than what happens in a blood choke, think FSH or PH or a BJJ rear naked choke. While this happens, your body will try to get rid of the SN NO MATTER WHAT (even with fasting/AE/benzo/paracetamol), mostly in the form of puking.
(Sidenote: some users on this forum said that the people in those videos, who suffered a lot by the looks of it, didn't follow the protocol and/or they didn't take benzos. We don't *know* that they didn't and as far as I know there is no reason at all to suppose that they didn't follow the protocol. For example in most videos they don't puke a lot, and there's a lot of dry heaving, which indicates AT LEAST fasting.)
SN's reputation on this forum is underserved and it's a 6/10 on the peacefulness scale on the PPeH anyway, which really isn't that good.
For reference, on a scale of 1 to 10 Nembutal is 10, morphine is 10, Sodium Azide is 5(!) and carbon monoxide is 8/10 (!!). But maybe 6/10 is acceptable to you. Since we have videos, it's undeniable that it can produce excruciating pain (in case it wasn't obvious enough that drinking SN isn't painful...), and the 6/10 shows that, too. It has 4 extra points of pain (out of 10, mind you) when compared to Morphine and Nembutal.
Again, I remind you that OP specifically asked for SN, not about its combination with a bunch of other drugs, like the whole fasting+AE+benzo+paracetamol thing.
Frankly, I even doubt it's 6/10. When discussing how painful SN is, it would be fair to only stick to SN. When you're discussing a drug you're discussing THAT drug, not the 5 other drugs you could possible consume that one drug with. I mean, by that logic you could say that falling on a bed of nails from 50ft is painless, because you can OD on an opioid before the fall. Idk, seems wrong to me.
TL;DR: how painful is SN? Probably a lot. The 6/10 on the PPeH is only true *when you include a bunch of other drugs*, the substance itself is obviously horrible and brutal. Definitely don't raw dog SN and think 100 times before choosing it as a method even if you have the other drugs at hand. We should stop acting like SN is the Holy Grail, it's giving a false impression.