See if turning photogrammetry off gives you the performance back. If it does what was the FPS on the ground with it on? and what is your Terrain LOD setting?
If the FPS is in the single digits it seems like the only thing to try is a clean reinstall of windows.
If the FPS loss is about half of what it was in the air for some of us removing the 3D photogrammetry cities parts of the World Updates brought the performance back. (the USA world update and Nordics can be left in full there, it has no performance issues). If you do this remember to restart the Sim after removing the objects before checking for any performance improvements.
Both can be read in detail in this thread.
If you don’t want to give up any photogrammetry in any areas the only thing you can do is to lower the Terrain LOD until you get the performance you want.