Hello, I've been having problem with Witcher. If any particle effect(dust/magic effect, etc) my fps will drop below 30. This happens with cutscene as well, while without particle effect on screen, I got around 40-70 fps. Is there a way to lower or turn off particle effect on this game? The biggest impact is barghest boss battle, my fps jumped down below 10 because all the barghest green effect. Turn down settings doesn't help much.

My rig
i3 2310M 2.1Ghz
GT 520M
4GB Memory
Win 7 x64

I know my rig isn't top notch, but I'm sure it is more than enough to play Witcher at least on medium.

try updating your grahpics drivers, that might work. you might even be able to run windows update, and choose all the optional udates too, and that could clear up your problem.

my graphic driver is up to date, since this only happens with Witcher, I guess it is optimization problem?

The reason is simply that the GeForce 520M is barely meeting the minimum specs.
To play this game with some particle effects you need to meet the recommended specs which is a GeForce 7800GTX or Radeon X1950.

The GeForce 520M is about 50% slower than these. The rest of your PC is well above the recommended specs, but the lack of power from the 520M will keep you down.