Cliffs of Dover laggy image

[CoolingThunder]

Hi,
I just installed CoD, and even on medium settings the image lags when flying over London. I have a pretty decent gaming system, here are my specs
Intel i7 920
ATI radeon hd 5800 1GB RAM
6GB DDR3-RAM
It runs any game on full settings and this game lags on medium settings, how could this be?
Regards,
Gijbels Jordy

EDIT: moderator, sorry if I put this in the wrong section, I think it belongs better in the performance thread, sorry