As far as I know, Civ4's core engine is quite old and not amenable to modding, and it predates multithreading being common enough to use. Some work has been done to try and get around this.
Leaving the "Enable graphics paging" and "Minimize AI turn time" options turned on in the General tag of the BUG Options Menu will help.
If you really want to, you can try increasing the value of NUM_CITY_PIPELINE_THREADS in A_New_Dawn_GlobalDefines.xml. I don't know if it will work, but I think it is part of a process that can use multiple threads to speed up the AI decision-making for what to do with its cities.