I have just done the video interview and I can confirm everything Dan said!
It's supposed to take 15 minutes but that's only because you watch three instructional videos. You get unlimited attempts on the practice question so you have plenty of time to fix your mic and camera quality (note that GR can't see your answers to practice questions!).
My advice would be follow the instructions as closely as possible. It sounds obvious but you'll want to make sure that you're dressed smartly, that you have a plain background (they specifically say no posters or messy furniture in the background) and that there is little background noise.
It's basically there to test your personality- you're articulating similar points to the ones you made in your written application.
Don't overthink it! Relax and use the 15 seconds prep time to take a few deep breaths.
I'd also make sure you're always looking at the camera (within reason). Remember that the video is just a medium for the interview- treat the camera like a real interviewer, so you're always making eye contact. This is something I could have done better.
There isn't much else I can add to what Dan said.
Dan, you're a lifesaver!