This game has been running perfectly at medium settings, and I started chapter 3, went outside and.. wow. I ran a benchmark with fraps, and it stays around 30fps everywhere, until I go inside somewhere, or to another area, which it goes back to ~60fps.
edit: inside places, I get like 80-100~ so FPS
in the town for chapter 2, I get a usually solid 60fps, and it rarely goes below 50 for more than a few seconds.
in this place, I get a pretty much solid 30 fps, which sometimes goes to around 40.

I've been looking for a solution for a few days, cause I REALLY want to finish this game, because it's amazing so far, but a few seconds of walking through this town is killing my eyes and making my head hurt, it's hell.

Turning every setting down does absolutely nothing performance-wise, as well as putting the resolution pretty low.

Doing the cpu trick, of unchecking one and then rechecking it after applying it, does nothing noticeable.

Rig:
Windows 7 x64 (installed like 3-4 weeks ago, too)
4gb ram
Nvidia GTX 260
Intel Core 2 Duo 3.0ghz

The Trade Quarter can make any computer cry :) There's a large number of actors and objects requiring CPU attention.

If you don't already have Vsync turned off, try doing so.

It was already off, but I've tried turning it on already, no differences.

Sometimes forcing vsync in the drivers doesn't work, and there's no in-game option, but you can use a 3rd-party program to force it in the Witcher. Check my stick thread Fixes to Common Problems for how to do that.

Enabling it won't give you more FPS but it will reduce tearing.

Oh, I've tried with both nvidias control panel and D3DOverrider, doesn't noticeably help any eyestraining or anything.

Cool, just making sure. Could have been you thought vsync was on but it wasn't.