Script AI heavily underperforming on non-asphalt terrain?

The Script AI feature is great if you're on asphalt, but the AI seems to mess up if you try to use it on any other surface. I ran three separate tests to figure out how the Script AI would perform against my own driving.

(All tests were performed with Safe Mode, and the only mod installed was the Motorsports Playground map.)
The AI slows down on bumps and sometimes slows to a crawl if the road is even remotely bumpy, despite having a vehicle that is clearly capable of quickly traversing the terrain. The AI takes absolutely no risks whatsoever, and even slowed down to 10mph in certain sections during the last test. What's causing this, and is it possible to fix??