Contrary what V12 (who seems to have a bad config somewhere on his side and generally seems intent in bad-mouthing FG) claims, the 'prehistoric' core of FG is very fast on a modern high-end system - I get a solid 60 fps with all graphical goodies enabled.
Without more info, there's various possibilities:
* the GPU driver may not recognize FG as application needing attention and starts in power-saving mode
* on a two-GPU system, the low performance GPU may be used
* there may be a virus protection/ monitoring software draining performance
* ...
Generally the idea would be to start from a
http://wiki.flightgear.org/Minimal_Startup_Profileand see what the performance is then. If that is poor, look for a systems-side reason why the OS does not allocate resources to FG, if that is okay, gradually enable graphical goodies and see where the problem occurs.