Looking at the various threads MSFS uses, the “heaviest” thread is reading and writing the hundreds of graphics and texture files needed. Reading and writing files has to be processed in a linear manner in a single thread. The next “heaviest” thread processes registry requests. Since the registry is one big file it makes sense to only have one thread for these.
Does the reading and writing of files limit (control) other processing? Yes, when other threads are waiting for a file to be processed.