It's uncomfortable but not painful really. I've had a few as I have celiac disease. Let's say that you don't have to worry about choking like with a gastroscopy, which I "luckily" had booked for the same day as the colonoscopy. I've never been sedated or had any anesthesia for either, it's really only the air afterwards that's painful, but that's not worse than when your stomach is upset during a flu or something.

It isn’t. Considered better too do it under a milder sedative in most cases I think as there are fewer side effects.

I checked and general anesthesia seems to he a custom used in some regions of America. I’ve had 3 colonoscopies and been awake for each one. They aren’t painful in the slightest and you get to watch them on screen! Anyone who gets one should talk to their doctor about sedation options.

I don't think it's a matter of being painful necessarily (though it might be, depending on what they do), but rather most people just don't want to be awake for what is at best a socially awkward experience. I've had several, and I'm grateful to have been unconscious. (They had to wheel me back in once without sedation to take another look around a biopsy site to make sure I wasn't bleeding too much. It was kind of interesting, but not terribly pleasant.)

My understanding is that in many places they are regularly performed without anesthetic so they must not be that painful.

They do pain research on people getting colonoscopies because it varies so much. A lot of people do find it unbearably painful. It's a relatively small fraction though.