If you are experiencing severe slowdowns, there are a few steps that you can take in order for the game to render smoother. I noticed incredible speedups when I started running Sims via FPSLimiter and rendering the game in Windows XP mode. Also, windowing seems to help a lot as well.
The slowdowns happen for several reasons: your computer can't handle the number of animation computations that Sims is trying to spit out, and your video card can't update fast enough, causing the hang time. Safeguard your videocard and you should see improvements.
Hope that helps.