I seem to be getting a gradual degradation of performance as the game progresses as well. I have been trying roadczar's ATI xfire workaround (
http://forum.1cpublishing.eu/showthread.php?t=23180) and gave a brief description in that thread.
To cut a long story short, the increased initial performance using the workaround has possibly made the gradual degradation in FPS more noticeable to me now.