some downloaded cars came with the config to edit in traffic_cars, like for exemple the CLK, that you have to add the following line:
<Car name="CLK55AMG" prob="0.043"/>
in the ...\City Car Driving\data\config\traffic_cars.xml
"prob" is the probability that cars apears in traffic...
but, how much cars you add in traffic, more lag you add eather.
i added about 6 cars and that made the fps drop down to almost 25. then i removed all added traffic, leaving only the originals, and the fps came back to more than 180fps.
and look that i have a very good pc