This is my specs:
Intel e7400 2.8ghz
4gb RAM DDR3
AMD radeon HD 6670 1gb DDR5
Windows 7 64 bit.

It's really weird that I found many youtube video claiming that worse PC than mine can play at even a higher setting and better FPS. That's why I bought the game.
I set the game with low dedault setting, and throughout the prologe, everything is perfect. I got around 40-50 FPS. Some stuttering, but it's unoticable.
However, when the chapter 1 comes, everything is messed up. The FPS drops to about 12-15, giving me headache.
Then I tried out some ultra low setting via google. Still, none works. No setting can take me up to 30 FPS. This is the lowest one I tried so far.

[Rendering]
AllowAntialias=0
AllowBloom=0
AllowBlur=0
AllowCutsceneDOF=0
AllowDOF=0
AllowDecals=1.0
AllowMotionBlur=0
AllowRain=0
AllowSSAO=0
AllowScatterDOF=0
AllowShafts=0
AllowSharpen=0
AllowVignette=0
AtlasTextureDownscale=0
CharacterLodGameplayDownscale=1
DanglesLimiter=0
DetailTextureDownscale=0
FoliageDistanceScale=0
Fullscreen=1
MaxAtlasTextureSize=512
MaxCubeShadowCount=1
MaxCubeShadowSize=256
MaxSpotShadowCount=3
MaxSpotShadowSize=512
MaxTextureSize=256
MeshDistanceScale=0.5
ShadowQuality=0
ShadowedLights=0
TextureDownscale=0
TextureMemoryBudget=256
TextureTimeBudget=10
UberSampling=0
VSync=0

[Viewport]
Height=720
Width=1280

Any ideas guys? Why is chapter 1 dropping huge FPS compared to prologe?
Tks in advance.

It's your GPU! Just to darn slow.