Just did a fresh install of Win 10 V1511 x64 from USB to SSD and boy was it fast. I wasn't timing it but I reckon 15-20 mins tops, why cant they do this with the upgrade path which seems to take about an hour?
I think you will find that it's because the upgrade has to backup the existing installation and check that all software and hardware installed is compatible.
Of course a clean install hasn't got to do any of this as its a pure vanilla build.
It took about 12 minutes on my main PC when I did this.