It's not a speed issue. They're in a blind spot. Your brain tells you that you can see everything, but it's incorrect.

My experiences have been under slow circumstances only. And that makes sense because the individual "lingers" in the blind spot (and perhaps matches your speed) if you're going slow. If you were going fast, the blind spot would be moving much faster and you would see them.

Not being able to see is a blind spot issue. Not slowing down because of it is… something else. Irresponsible is the most polite I can come up with.

My experiences have been at around 5mph. Again, it's not speed.

If you watch the video in the post you will understand. The person is just invisible to the driver and they're travelling around the same very low speed.