4x FSAA in Direct3D Slows Down Doom 3?

[First of One]

Wonder if this is a Cat 5.4 driver bug; I don't remember seeing it in 5.3:

With AA set to "application preference" in both the OGL and D3D control
panels, and no AA set in-game, I get an expected 51 fps in Doom 3 Demo1.

With AA set to "application preference" in the OGL panel, but "4x" in the
D3D panel, I get only 26 fps. However, there's no change to the image
displayed. Why the f--- are changes to D3D settings affecting Doom 3?

Anyone else experiencing the same problem?

 

[Bratboy]

What happens if you set cp to app and select 4xaa in game? Do you still see
a drop?

 

[First of One]

When I did that, anti-aliasing can be seen in-game, with the corresponding
drop in framerates. However, 30 fps average in Demo1 translates into low 20s
in certain spots, not smooth enough.