Lag when opening map

So I have an odd problem, when am playing COH I have little to no lag in most missions. I have the graphics settings turned down not all the way, but low enough so i don't see a framerate drop. My issue only occurs when I enter a large map, such as a safeguard mission or certain mission maps. It will play perfectly fine with almost zero lag but if I open my map it will get basically unplayable its so laggy. After opening the map, nothing I have tried will stop the lag short of re-zoning and not opening my map. Anyone got a clue as to why this would be a problem? I have tried everything I can think of to fix it, but short of not using my map in those missions I have not found one.