There are a couple known issues that could be at play here, the most likely is this existing report:
https://forums.flightsimulator.com/t/main-thread-loads-up-more-and-more-and-doesnt-recover/729018
This is located in the Bug Reports Category, under Install, Performance & Graphics.
You may want to browse through that section for others that may apply.
A fix for this is in the beta test currently underway, so some measure of relief will come when that is released to the public.