From what I understand: 3D performance in general needs to be improved. Even if you’re not rendering them, having tens of thousands of nodes in the tree kills performance. The global illumination used in Godot is also really taxing. Terrains are an extension rather than built-in to the engine and probably doesn’t have feature parity with other engines’ terrain systems.

4.4 did a lot to improve things but there’s still a ways to go before big open worlds can happen.