Sorry man there isnt a way to do that. Therefore this topic needs to be moved to the game section. I dont know why it would lag when its not active, but that would be a good way for NCsoft to help stop people who do what your doing. (I think they frown on it). But it may be just a problem with the game itself to be fixed in a new patch.
And the lag in the inactive window deals with how MS timeslices the proc. Active windows always get higher priority than background windows and tasks. So, the inactive window has to compete with all of the services and other background activities on your PC while the active window gets more processing time. A dual core proc will help some, not because it will raise the priority of the inactive window, but because it will give you more processing power for that inactive window to share.
I dont agree mr_fnord is correct.
I have a xp2400 (had at the time) and ran 2 copies of guild wars, Lineage2, and WoW. This is not your issue, you need to post your questions on NCsoft form to get this resolved,