Vanilla cars get better FPS than automation cars. Explanation?

Automation cars don't even have an interior, they're not particularly detailed, even the really good looking ones, but some of them lagg on my system where I can load up any highly detailed vanilla car and it'll get rock solid framerate. Is the engine not optimised for certain uses?
With Automation cars, It seems the rims and number of fixtures affect the framerate greatly, but I don't understand how. Anyone can explain it in detail?

 

High number of polygons = low frame rate.

Automation body meshes are insanely high-poly. This is because they were designed for Automation, not BeamNG, and exporting an Automaton car to BeamNG doesn't optimize the model or lower the polycount in any way. Every fixture adds even more polygons—think of them as cinderblocks added to a boat that's already too heavy to float.

Plus, if I remember correctly, the detailed engine is modeled even though you can't open the hood to see it—so go ahead and add a two-ton wrecking ball to that heavy cinderblock-filled boat.

 

I get Hardly any Performance dips with automation cars, It might be because i dont nuke them with a gazillion fixtures.