Interesting, didn't know that! I think I have my whole 5km+ track in, like, 5 total pieces. I've been meaning to update the phys mesh anyways, so I'll try to integrate this suggestionThe actual limit is 65535 vertices or faces per object, but for any physical mesh (roads) you want to keep it under 20k and limit their total dimensions to under 500m. in terms of ingame performance it's more about keeping it under 2 million visible triangles than a total number. (on most vanilla content there's a million or so in grass but it all stops drawing 30-100m from the car so you only see some of it)
Disabling fbx settings can help import speed, especially animation options.