I was in the process of getting the game when I finally took a note at the SysRegs, and they have a very high CPU demand listed as minimum. Forum discussions seem to confirm pretty noticeable performance problems even on high end machines, most likely CPU related.
My CPU certainly is old, but runs current racing titles with plenty of AI and physics, runs Witcher 3 and Dying Light with large crowds active etc. It appears to me they got their xeno horde AI wrong and bigger amounts of enemies just bring systems to their knees. Their system requirements already were steep because they knew they had bad optimization... and now it's not really working really well for most people.
I hope they can fix it, because from the gameplay that does work people seem to confirm it's a really true to lore and visually gorgeous WH40K game.