Originally posted by SiEgE:

You can still optimize LODs, lightmaps and etc.

What I mean is that if you know that your game has a certain forest area for example, you can prepare the forest to be viewed from a distance as a singular unit, which opens up a whole world of optimization techniques you can use to make it efficient. You can bake any amount of information you need about displaying the area efficiently. By contrast, a procedurally generated forest looks different every time. The result only really exists when its generated around the player and not before. Issuing draw calls for each tree in the forest for example is very expensive in terms of performance, but merging geometry together dynamically for faster drawing is a very difficult thing to achieve when you have to do it strictly in real time.

Same thing with lightmaps. You can bake the lighting if you know which objects and lights exists and where, but since you don't know in this case, you cannot prebake information about them.

I reckon most people don't think about these things, they just take notice that their favorite triple A game runs much better and looks better too, so its disappointing that Valheim is not on par with them. But there is a reason why most procedural games look more modest and run worse.