I suffer from the bad framerates, too, could do nothing more than leave my report on it in the tech forum on it...
But back to the engine: is it a problem of the crystal space engine or of your your implementation or usage of it?
I hope it?s no general problem (with every game using this engine having these problems), then I guess it still lasts at least half a year until CS has strongly evolved and I can play PS in a reasonable way

Or can the Planeshift team itself do something about it?
Whilst looking at Sourceforge, I found some of the other zillion rendering engines like
Irrlicht,
Neoengine or
Ogre...
Don?t know much about game engines, but could it help to implement features of these or address problems of the current one?
Currently even heavy fog would make me more happy when looking at my fps...