Have you tried other 3D apps and see if the slow down happens?
If it does then driver and/or the card has the limitation.
How am I sure what constitues a 3D App? If a video player doesn't count, then maybe Marine Aquarium is one of the few that I have. Actually, I have Goldfish Aquarium too. I could try it with that. Otherwise, DXDIAG shows no problems with 3D support in testing, and as I mentioned above, I
did do a driver update very recently.