I’ve been trying to get this fixed for ages now. A Dev was even looking into it for us at one point. There are dozens of threads about this and it seems to randomly get triggered for absolutely no reason with their patches. My wife had the same issue for around a month, then the summer solstice patch fixed it for her. The big quest update with orki’s challenges fixed it for my mate. The DLC is what caused it to trigger for me, it was fixed for me until the new balance beta patch when it was triggered again. The Devs don’t seem to have any idea what’s causing it at this point.
In order to fix it, switch the game to DX11 and that will stop the screen freezes. But you will still get really bad FPS during hordes.