Weird performance behavior on a low-end system

[Bucic]

Mobility Radeon HD3650, 1680x1050 native resolution, Catalyst 13.1
8 GB RAM
C2D 2.5 GHz
Linux

Consider this:
Preset 1:
SSAO: minimum
Shadows: medium/medium
Posteffects, Smoothing, Vsync: disabled
All other: enabled or high

Preset 2:
SSAO: disabled
Shadows: disabled
Posteffects, Smoothing, Vsync: disabled
All other: disabled or lowest

(1) Now, the difference between performance for the two presets is not noticeable. I bet it wouldn't be noticeable either if I added some post effects to the Preset 1

(2) Also, in the corridors (small rooms) I get ~20-25 FPS while in big rooms with lots of lights I get mere ~10 FPS.

Any thoughts on that? How is (1) possible? Can the cause of (2) be slightly eased by fiddling with config files?