I started experiencing the same problems last night at about turn 235+ in my first game since installation. All of a sudden, the game became laggy, choppy, slow. I alt-tabbed out to see if something else started running(damn McAfee automatic update or something). It wasn't McAfee, but Nero(and old CD burning program I don't use that's been on my machine from when it was built). One of the Nero exe programs was taking up huge amounts of memory and cpu usage, which was strange as hell, I didn't start the program). So after shutting it down via task manager, a different Nero related exe started hogging resources. I finally just uninstalled Nero and have yet to play G+K yet to see if that fixes the problem.
I am playing on a duo-core Pentium 3.2, 2 gigs of ram, using Windows XP 32 bit. (I know, I know, but when my machine was built Vista was laggy and XP worked just fine for Civ and WoW, the two main games I played. I never upgraded to Windows 7, figuring I'd just do that when performance dictacted a whole new machine upgrade.) I am wondering if this is a bug related to XP. I have no clue what could cause Nero to suddenly start up and start hogging so many system resources.