Windows distributes workload to the cores, not MSFS. If this is a workload distribution, it would be a Windows problem, not an MSFS problem.
The problem is the nature of the workload. For some reason, MSFS is reading a lot of lies into memory at the same time slowing everything down.
Yes, the FPS counter shows it is limited by the main thread which is correct. But what is limiting the main thread? The FPS counter doesn’t show anything that is limiting the main thread. Actually, the FPS counter doesn’t care what is liming the main thread. All it is saying that the FPS is low because the main thread isn’t sending frames fast enough to the GPU. In general the primary job of the GPU is to make the graphics frames and send them to the monitor. The CPU (main thread) takes care of everything else inside of MSFS. The FPS counter doesn’t have any visibility into “everything else”.