what kind of thing? turning out of a junction. yea, i do it quite regularly. i thought it was going to be a clip of someone coming out and driving at 5mph. the video clip appears to show a car coming out of a junction and albeit a bit slower than you want, them getting up to speed. any more of the clip that shows the driver maintained their slow progress, in which case you have a point.
One of the first things you are taught when learning to drive is to not emerge from a junction if doing so would cause another driver to potentially have to brake, stop, take avoiding action and you should quickly get your speed up to that of traffic travelling on the major road. As I was over a crest and into a downhill I had to brake a fair bit to bring my speed down from around 50 to IIRC about 25MPH and they took their sweet time getting up to speed - they didn't significantly increase speed after that clip for quite awhile. Then ironically when we got to the next village they continued through the 30 at about the same speed as they were doing in the NSL while I dropped down to the speed limit :s
I don't make a habit of pulling out on another driver but occasionally we all make mistakes and/or if it is busy sometimes you have no choice but to be pushy or you'd sit there forever - but if I do I at least make sure to get going once I've pulled out.
EDIT: My dashcam due to the mounting position and lens angle kind of hides speed a bit as well especially changes in speed - it was a fair bit more dramatic change than the clip conveys.
They didn't even stop at the junction, they can't have been looking properly given the road furniture in the way and the fact you emerged over a crest. I have more issue with that than their speed although I also see your point there.
Yeah - I'd hazard they'd have still emerged if I'd have had to stand on the brakes to avoid hitting them as well...