The Civ4 core game dll was its own bottleneck. It's is terribly inefficient c++. Years ago, I was working on my own mod to optimize it because I got tired of waiting 15 minutes for the AI to complete a turn on a huge map late game. It was not hard to get it down to just a few minutes. I think it could be used as a poster child for a code base written by someone expecting the optimizer to work miracles. Lots of macros hiding expensive call chains used in tight nested loops, repeated calls in loops to get the same item that should have been called once outside of the loop(s), etc.

Python was not the bottleneck, just the scapegoat for badly written code.