I have been fighting through the FAQ and virtual memory settings and all of the community suggestions on Ashes to no avail the last two days. I found the magic bullet that fixed my issues...
Patience.
I would launch Ashes in either DX11 or 12 and would be greeted by a black screen or a white screen with a cursor. I'd let it sit there for a good 30 seconds -- far more than I would expect for a system as powerful as mine, and far more than I have waited for any other game. It turns out, I just needed to wait a little longer.
How long? Like "I finished a game of Yahtzee with Buddies" long. After a few minutes, the game roared to life and I was greeted with the intro. The game now loads quickly in DX11 or DX12 mode. Whatever is going on in the background just needs your patience to succeed. Who'da thunk it?