workers are a huge culprit. The computer puts a lot of "thought" into where to send workers and what needs to be improved. When I used to use automated workers, the lag was rediculous; when I stopped automating them, bing! Problem mostly solved. The AI has tons of the little buggers in the late game, though, and they're all just as good as "automated" and so create a big draw on your computer's processing power.
So in summary, if you want faster IBTs, abduct as many AI workers as you can