I absolutely LOVE the Sims 3, way more than 4, but if there's anything that drives me away from it after a while, it's how badly the game can run at times. I know there's been lots of talk of the Sim...
I fear the slapped together spaghetti code igazor mentioned has a lot to do with it, in combination with often rushed worlds with routing issues that are absolutely unnecessary (which becomes clear when you fix them).