Actually If I recall right there used to be a video out of someone using an XBone, before said video was pulled, and while it may not be as fast as the mockup it was far from slow.All a mockup done in After Effects. Word right now is that the dashboard is nowhere near complete.
Think of something closer to the Wii U dashboard before the update last Spring: slow slow slow
But who knows how the finished product will be, gotta wait and see. Until then all we can do is speculate.