Skyrim

I'm afraid that on XXXL maps, it's impossible to find a balance between performance and the number of objects – something will inevitably suffer, and the game itself struggles to handle such large maps. Even if it doesn't lag, there might be problems with saving the game later.
So, either you need to do an extremely good job with game design (using few objects but placing them very strategically so it's not noticeable), or you should switch to a smaller map size (in my opinion, this is the optimal option, although it's a pity to discard the work already done).