Anything below ~ 15 fps can probably be considered to be hardly playable or at least enjoyable. FlightGear's framerate is influenced by various factors, among others:
* scenery complexity * cockpit complexity (2D vs. 3D) * environmental options (i.e. visibility) * FDM update interval * AI scenarios * Aircraft speed (scenery paging)
You can usually disable pretty much most features that may be framerate killers, here's a collection of settings that you may want to try, please note that some of these options will only take effect if they're passed at startup time and won't affect the game at runtime:
* LOD * AI traffic * 3D Clouds * ...