We're aware that the trees are heavy, that's something we'll change for the next version.
All of our camera processing is done in _physics_process, could you maybe send us a video showing the problem? We haven't been able to reproduce it.
Our loading is a bit complicated, but I'll try to explain. First we create a cache with all the materials in every level that will be used, then when you go to a new level, we use another thread to load it. We place it in the correct position in front of the player, where it would be if everything was loaded at once, and then we free the previous level. Check out the background loading tutorial in the Godot's documentation.