Nah mate, there's a time for taking the p!ss, but someone asking a serious question should get a serious answer even if it has been posted before as sometimes there are just too many search results to trawl through for the answer.
Well it depends. If there are too many cars, it's usually quite low. And if it even starts to rain, the RL engine has to render many reflections and refractions. Usually it's best at night, when there are no other AIs and the visibility distance is pretty low. You know what is funny? When I turn on the radio, it doesn't lower the FPS at all. Not that I'd complain