I am pretty sure that the main culprit behind the poor performance is that they use some wholly dynamic lightning solution with nothing pre-calculated or preset. This technically means the lighting is more true to life and the level designer does not have to light the scene beyond just placing light sources. Problem is that means the engine has to basically re-calculate the lightning for every frame and that is just stupidly taxing on any system.
You don't need to be pretty sure. It's called Realtime Global Illumination.. and I'm believe they used the same tech in Fallout 76.
It's certainly expensive to calculate real time and really comes down to what their update interval is (I believe they update every in-game hour which isn't that bad actually).
AFAIK This accounts for their sunlight direction which I believe their engine can only support one source at a time.. So no Galaxies with two suns on opposite sides of a planet.
This isn't some new fangle dangled technology though.. They used it in FO76 and it's been in Unity and Unreal for years. I'm not sure if they in-housed it or licensed it though.