I have a Space Mouse Pro.
I want only know if also you (who of you owns it) have seen a slowness in big assemblies (more or less more of 200 pieces).
I know that even without 3d mouse, in rotate/move those kind of assemblies, the PC is slower, but if I done that with the standard mouse the movement is faster than with the 3D mouse.
So it is not a problem of the 3d card, I think.
I use WF5 M170, windows 7 pro 64 bit, 3d card nvidia quadro k4000.
Sometimes the graphics are trying to do hidden line removal if you are not in shaded mode.
Then the Space Mouse can hang while it is updating graphics.
For the most part, I don't see any difference.
The only other feature that is GPU intensive is transparency. You can toggle this off in the render tab.
Otherwise, I don't know what could be happening. I use the navigator and have never felt that it slowed the interaction with the system.