The way I see it Civ 4 BTS has 2 problems. First is overall lagginess (happens usually with mods and late games) and the second is stupid AI/"some other stuff" that can cause severely long wait time between turns (can only be fix via modes).
If you got some extra unused ram you might be able to reduce the lag by "cheating" a little bit
. Take a look at this guide which I wrote some time ago. It basically eliminates the hard rive as any bottleneck for the game
http://forums.civfanatics.com/showthread.php?t=414845
Enjoy