You dont mention your computer hardware so that could be the cause, you need a powerful CPU to run all the AI and maintain a reasonable framerate.
You're also using AI mods like SAIN. These can have adverse affects on your FPS.
I have an [email protected], RTX-2080Ti, 64Gb of DDR4 memory, and an an NVMe ssd. ( Rendering at native 1440p ). This isn't a hardware issue as I stated in the post the game runs perfectly everywhere else even on a far more taxing map like streets. Also, with task manager open I can see there isn't a spike on interchange.