First of all, Cities XL 2011 captived me! It's a great game. I have the same lage problem. I have a high-end configuration with core i7 980X CPU with 12 threads, 12 GB RAMS, and GTX 480 videocard.
I have 500.000 citizens, and the the frame rate drops dramatically, and it make the game unenjoyable.
I just share it for Focus Interactive guys, to make a patch, with that the game can use all the hardware resources.