Yup, it's boxes that need to run logic every frame and don't just get removed from existence if you turn the camera away. It's doing a lot more than just rendering.
Compared to something like God of War that is trying to turn off anything not in the currently loaded level chunk (which of itself has mostly static assets), a simulation game needs to spend its budget moving around assets all over it's scene, or at least prepare approximations for where and what an asset is doing once it comes back in view. This is why such a game tends to be more cpu intensive. But I wouldn't be surprised if the GPU is being used for compute in such a game.
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With all that said: I haven't played but to answer your specific question it sounds like the game has a very heavy and/or suboptimal GI scheme. People here have said to turn off GI and volumetric and they get much better performance. That kind of dynamic lighting may be the main culprit for a game already taking up a lot of cpu/GPU budget from the above factors.
Compared to something like God of War that is trying to turn off anything not in the currently loaded level chunk (which of itself has mostly static assets), a simulation game needs to spend its budget moving around assets all over it's scene, or at least prepare approximations for where and what an asset is doing once it comes back in view. This is why such a game tends to be more cpu intensive. But I wouldn't be surprised if the GPU is being used for compute in such a game.
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With all that said: I haven't played but to answer your specific question it sounds like the game has a very heavy and/or suboptimal GI scheme. People here have said to turn off GI and volumetric and they get much better performance. That kind of dynamic lighting may be the main culprit for a game already taking up a lot of cpu/GPU budget from the above factors.