That nVidia chip is a hybrid - it works in tandem with the Intel GPU on the processor.
Which explains the slow down after some time.. The Multithreading of the game is using more of your processor making less of it available to help the GPU. It probably handles other games okay.. but most games don't utilize much processing power. High CPU usage is going to have a direct effect on your graphic performance with that card. (something benchmarks usually ignore when testing making them useless when judging the performance of this type of GPU)
Under minimal load its similar to the 7900 as heaven fall mentioned. But under more load its performance is going to degrade due to the hybrid design.
(edit if you have pedestrians enabled that causes a huge performance hit turn those off. I lose about 1 or 2 FPS for every building Built with pedestrians on)