I had an idea while hanging out my washing that made me rush inside. I suspect that the problem with mods is that civ4 loads every bit of artwork and xml into ram; even though in a mod it might only use 1% of it. If you have 100+ civilizations with levels of unique buildings and units, this is an incredibly wasteful way to do it. It makes sense to choose what races etc are in the game... before you load the artwork and so on.

This is not a problem in vanilla because you only have a few races, maybe one unique building and unit, a few religions, and so on, and the percentage of 'fat' is low. In Visa it is enormous. Maybe a pre-loader is needed? Since i doubt firaxis is going to open up the code enough for us to fix it?

Essentially it would be a program that creates a temporary customised mod, by copying the relevant artwork etc into the folder. Thus, if aztecs are never used in the game, then aztec units, buildings etc do not take up resources.

I'm smart enough to see the answer but not enough to code it. Gah, it's like trying to eat steak through a straw.