Was UAC/DEP disabled when you installed?
GoG's installer doesn't like those things, so the game may have been corrupted. I find I have to turn UAC off AND install as admin to get the game installed right on my Win 7 rig.

I did. I recently purchased the game and ran into the same issue: shortly after starting the game, it would slow down to the point of being unplayable. After reading your post, I tried turning off the Logitech controller program and unplugging my Logitech Rumblepad 2, which fixed the problem.
I have to thank you for the suggestion. I would have never figured out it was my controller causing the problem. :)

Same issues here under Win7 64bit. I tried all the standard procedures mentioned in the Common troubleshooting tips, also the ones mentioned in this thread and although the slowness seemed to be slightly less severe when I unplugged both of my controllers and the shutdown the logitech gaming software but it didn't help much at all. :S

I might try to run it in XP 32bit under virtual machine.. In fact I think I'll try that now.

EDIT: No cake. :(

Same for me, I've tried different compatibility settings and created a shortcut with "-nocputest".
Game works fine for about 20 seconds then it turns into a stuttering slideshow both in-game and in the menus.

System:
i7920
560ti
6gb ram
W7 64



Edit: I have contacted support, I will post the results here.

Edit: We couldn't find a solution but GoG was good enough to offer a refund.

Unplugging controller works perfect for me :D

Theres my solution for this problem :)

www.gog.com/forum/expendable/my_solution_for_controller_support