Why is screen res the only setting that affects my fps?

[Kantorai]

I have an old laptop Vaio FE31H with a Nvidia GeForce Go 7400 GPU. In order to play Amnesia I have to lower the resolution to 640x400 (native 1280x800). Then I'll get a frame rate of about 20 fps which is playable (kind of). But I see no difference in fps if I change other graphics settings like texture res, shadow res, shadow qual to high, medium or low. Can't see any difference for texture filter, SSAO sample or Parallax either (haven't tried all settings for sample but I keep it low). If I change screen resolution to lets say 800x600 fps will drop to 10 and higher resolutions will result in even lower fps. All this is in fullscreen with anisotropy off and smooth edges off.

Shouldn't I see at least some improvement in fps with lower texture and shadow settings? Is Amnesia utilising my Nvida Go 7400 properly?

The launch window says "Video device: GeForce Go 7400/PCI/SSE2" and the hpl.log says:
Setting up OpenGL
Vendor: NVIDIA Corporation
Renderer: GeForce Go 7400/PCI/SSE2
Version: 2.1.2

Your help or comments are most welcome.
/Kantorai