Hi, yes, the same thing happened and that was around 2 years ago, shortly after MSFS was released. I even had it with a bush trip I created myself. At the start of a random leg the fps just tanks and the mainthread gets ridiculous high as seen in the screenshots. When I finish that leg and start the next one - the fps is back to normal.
It’s not a graphics setting issue - when flying the same leg without loading that bush trip the fps is perfect. And when I do a leg where the fps tanks it doesn’t matter if I lower my resolution to Lego quality, mainthread remains above 60ms. It’s just a superweird issue.
As I said I saw messages from others posting exactly the same issue, unfortunately I haven’t been able to find a fix or an explanation how that mainthread can get so high.