Bad graphics performance under KDE
When starting Flightgear under KDE, the graphics performance is very bad, i.e. low framerates, while running Flightgear in a different window manager shows better performance.
It is likely that "compositing" is activated. KDE includes a large number of various effects which use OpenGL. This might degrade the performance of applications that use OpenGL heavily.
For KDE 4.9+: You can import a so called window rule:
Save the following as flightgear.kwinrule, then open KDE's System Settings, go to Window Behaviour, then Window Rules. From the right select import rule, then navigate to where you saved flightgear.kwinrule, select it then press Open. You will be returned to the Window Rules tab. Press Apply and then close the System Settings window.
Next time you start Flightgear (or osgviewer for that matter) kwin will block compositing as long as Flightgear is running, then restore compositing when you close Flightgear, regardless if you start Flightgear in windowed or fullscreen mode.
[Application settings for osgviewer] Description=Application settings for osgviewer blockcompositing=true blockcompositingrule=2 types=4294967295 wmclass=osg osgviewer wmclasscomplete=true wmclassmatch=1
Solution for older KDE versions
Disable compositing permanently or temporarily in the KDE settings, via the assigned hotkey (normally Shift + Alt + F12) or via a script.
With compositing disabled, KDE desktop effects don't work anymore.
WARNING: This method is obsolete, and is only left here as a convenience for those using very old versions of KDE.
This example by i4dnf deactivates compositing, starts Flightgear and reactivates compositing after Flightgear ended.
#!/bin/sh qdbus org.kde.kwin /KWin toggleCompositing && /usr/games/bin/fgfs $@ && qdbus org.kde.kwin /KWin toggleCompositing