Update: It seems one of the causes of this slowdown was a memory leak in the music playback, unrelated to nvidia cards. This has been fixed and a few safety measures have been applied to improve performance overall.

Old entry:
I have had a few testers report performance issues after running the game for 30 minutes or so, this seems to only occur for nvidia cards under Windows. The quick fix for now is to restart the game, that will bring your framerate back up.

I am working very hard to find a fix for this as soon as possible.