The current problem with flightgear/simgear is the handling and loading/unloading of 3d objects and there impact on the simulation.
This is due to flightgear is current a single threading program and there are points in the code need be optimized one thing is that we can do as a start is splitting FDM/aircraft/main loop, audio/radio navigation, and the rendering subsystems form each other.
And the property tree run as there own thread.