I know this thread is SUPER old.. But I just found out what was causing the ridiculous lag in my game. I have a Nvidia 960m with an i7 processor.
Several days ago, I was only experiencing lag in hidden springs. Then I downloaded a few Sims from a YouTube channel and then all my towns began lagging every few seconds.
So I went and checked my Saved Sims folder and lo and behold.. These new Sims were about 150,000 KB versus my other saved sims which are about 35-120kb. I deleted the new Sims and my game is back to normal completely.