Hello, as the title says I have a problem with the game performance, I hope someone can help with that.
When I launch the game it runs perfectly smooth for 5 to 10 min (with 70-80% cpu core1 used).
But after it goes up to 100% usage and the fps drop over time, after 30min I have something like 5 fps.
Well that's all… if someone has a clue about that, thanks.

What are your system specs and what OS are you on?

Sorry I forgot.
Q6600@3.4ghz
4go ddr2
radeon 4870
Windows seven home premium 64 bits.

I have the same problem :-/
Phenom II 3.2GHz
4GB DDR3
Radeon 5870 - catalyst 10.3
Creative SB X-Fi Player
Win 7 Ultimate 64 bit

You are not alone. I have the same issue. Haven't yet found a solution: http://www.gog.com/en/forum/outcast/framerate_woes/_/1
Running:
i7-920 @ 2.66 GHz
6GB DDR3
EVGA GTX 275 (Not that it matters - game is software rendered entirely)
Windows 7 Professional (X64)

Have any of you guys tryed setting cpu affinity to a single core?

I had the same problem. Launching the game using the Loader.exe fixed it for me.

That worked for me too. Thanks!
my specs:
phenom ii x4 965 BE
3.25 gb ddr3 1333
geforce gts 250
audigy 2
windows 7 professional 32bits

I tried yes but it's even worse.
Doesn't work for me :(

If you try running it from the loader, set the compatibility mode on that to windows xp sp3. To do so just right click the loader shortcut and then go to properties, compatibility tab and you can set it there.