Yup seen that, had to wonder whats going on with system ram, and not shocking really as vega is running on amped up fury drivers and its not got rasterisation, rpm, hbcc, pd/ps etc enabled yet, all the fun of crap drivers.
Pay attention to 0:38 as he turns the right hand corner, 0:53 as he turns to the left, etc. (there are some other instances but can't be bothered to list them all) nVidia seems to be using lazier garbage collection and some kind of pre-emptive shader loading while AMD is only loading in data as needed which is causing momentary hitches.
That is quite a big difference in system memory though I wonder if they are both using same feature level, etc. there must be something else going on even taking nVidia specific functionality into account.