Try using the global reset command (resetsim *). I've found it helps with this kind of lagging.
It DOES but it also leaves cars scattered around the city. I tested this in one of my games, and it seemes if there is a sim (townie, whatever) driving a car when you do global reset, the car is left on the street and there's no way to remove it. Just a warning, I've had this car thing happen in 2 games where I've used resetsim * several times.
I found a work around for this! Or maybe this could be a proper workaround:
I went to Edit Town, and saved the household to the bin. The game gives a pop up about this resetting every sim's actions and queues (isnt it the same as resetsim *). It loads a while and tadah, the lag was gone, and for now Ive found no stuck cars.
(I get this same lag in Riverview from time to time)