We don't know how to fix it, the scenery is fully SDK compliant and has been compiled with the MSFS own tool, and it's an officially supported and documented type of scenery. It always worked perfectly fine until a certain sim update, and nowhere in the SDK there is any indication we should change something or do something any differently to support possible changes in the simulator.
The only generic answer we got from Asobo, is those kind of scenery objects weren't supposed to do entire countries, which doesn't explain why it worked perfectly fine before but there are no alternative in the SDK, and the only unofficial alternative (the .CGL file format) doesn't have any official tools and can't be used to do small countries bordering several other countries, because the minimum size of a scenery made with using the .CGL mesh format is way larger than the entire Switzerland, and it will surely conflict with other mesh from nearby countries.
We *suspect* that, the simulator is trying to load two meshes at once, the default one and the custom one, instead of just taking data from the one of higher priority or higher quality and, after the default mesh in the area got an increase in resolution with the Switzerland update, it's possible the simulator just can't keep up with all that data.
Some users reported a marked decrease in stuttering by switching the audio device in the sim, then switching it back again, before every flight. This kind of solution leads to believe there's some kind of bug related to handling of memory and/or resources as well, which might be cleaned up in some way when triggered by an audio device change.
