I've seem complaints all over the place about this, only to be shot down by "get a better computer" or "set your mouse to move faster" answers. There's nothing wrong with my computer or its GPU; it can play large battles on highest settings without lagging and plays every other game very smoothly. Increasing the mouse movement or window scrolling speed won't help.

I was playing a campaign (as Armenia) in an unmodded RTW from Steam, and one day, the campaign map just started lagging REALLY badly for no apparent reason. It was perfectly fine before. I hadn't changed anything; it was still bad in other campaigns and after I wiped my hard drive, reinstalled Windows, reinstalled RTW, and defragmented. It's so bad that I sometimes enter battles just to escape the lag because the game runs very smoothly in battles.

But it works fine on low settings on an old laptop of mine (not the computer I'm using)! So there's some bug in the campaign map related to my GPU. It lags just as badly if I set the graphics low. My GPU is a GTX 650Ti Boost adapted through VMWare Fusion, which should be easily powerful enough. I tried updating the drivers, no effect. Does anyone know of a fix?