One thing that's very apparent is that with more traffic the simulation slows down while the framerate isn't (so all cars go in slow motion, even though I'm at 3x speed). This means that it's severely CPU-limited.

I don't know how multithreaded their simulation is, I have a 5950X with 32 hardware threads. Maybe an upgrade to the new generation of Ryzen CPUs that are going to come out around February could help.