That’s happening to me too, which is scary because I’m playing a legacy game! (Basically, trying to get to 10 generations with 1 family).
If you haven’t already, I suggest the NRaas mods (http://nraas.wikispaces.com/). They help clean up unnecessary things like TVs on in unoccupied houses and extra cars in parking lots, which makes the game run faster and smoother.
Oooohhhh! That sounds mighty handy, thanks so much! :O You really do notice when the game slows down or speeds up again depending on the amount of things that need to be loaded. >_< Gah! Why do NPCs have memories too?!
Not great design, if it slows down that much as the save file gets larger… *grumble grumble software engineering grumble*
I know! And there’s no way to disable it. :< Really bad design indeed, considering it's a game that's supposed to run on most PCs smoothly.