The "decorative" cars driving around the town are deleted when you exit the save. When you reload, they'll be spawned again, a bit more every Sim day. So, whenever you find that your game is lagging, save, exit and reload.
Another big cause of lag is some mistakes in the routing of the worlds. They are many of those, around 10-20/world, and the situation is getting worst with every update of the code (EP, SP, patch). The only thing you can do is to reset all your Sims by typing
resetsim *
in the command line. This will send everybody back home.