kar82: My problem was "similar" (long loading time and chrashes), after I update my Win 10 Pro to the new Version 1511.
Then I update to the newest AMD beta driver (15.11.1 Beta), but the problem was the same.
The next step was the solution (I'm using the GOG Galaxy Client): I uninstall the DLC "Game Overlay" and deactivate the "In-Game Overlay" in the options of the Galaxy client.
After that, the loading time was like before the Win 10 Upgrade and I haven't any crashes anymore. I've playing about 3h without any problems!
But why this problems after the Win 10 Upgrade? Is this a bug in the Galaxy Client or in Witcher 3?
My System: i7 2600k, 8GB RAM, Radeon R9 270X
PS.:
I haven't tested yet to play The Witcher 3 with installed DLC "Game Overlay" and starting the game without the Galaxy Client.
You are my hero! :D It worked. Thank you.