I just wanted to play witcher again and encountered some issues.
Most of the time I am above 100 fps but sometimes I get a lag or slow down and it drops to 30s or 40s.
Not for long, maybe 3 seconds max but it is annoying since I played it years ago without any problems.
Back then I had an i7 920 and a GTX 460.

Now I have an i7 6700k and 2xGTX 1080 SLI.
I disabled the second card already since the light sources shine through walls.
I tried enabling vsync via the console panel but none of the settings work.
I disabled DSR, and the overclock of my 1080s.
I even alt tabbed and set cpu priority to very high and set core affinity because sometimes I see 100% CPU usage on Core1 and almost 0% on all other cores.
My GPU sits at max 23%

I uninstalled all NVIDIA stuff and reinstalled the driver, but without audio, geforce experience and 3D.

It can't be my PC since I currently also play Witcher 3 in 3440x1440 with ton of mods and extreme ini settings.

PS: I had to switch to 1080p for Witcher 1 since the cutscene don't play in ultrawide.

Have you tried turning off Environment Sounds?

I had the same problem, the only way I was able to get vsync to work in The Witcher was by using a tiny program called D3DOverrider that came packaged with RivaTuner. I'm not sure how easy it is to find now. This probably won't fix your slowdowns but it might be nice if you want to eliminate tearing.

D3DOverrider won't fix the tearing any other ideas ? why did they removed the vsync option ?