Found out why Oblivion slowed down
When I first bought Oblivion it ran very well on my machine. Later I installed Shivering Isles and another expansion pack. Since then I noticed huge slow downs.
However I must have also done some Direct3D programming in between installing the packs and installing the game. Found out that all of my Direct3D settings were set to use the debug runtime with shader debugging and validation enabled. Debug output slider was set to about half. Debug runtimes were being used for every component of DirectX.
I guess using your gaming computer for game dev is not such a good idea. :D :D