I just want to know why it lagges. My macbook pro is pretty good and it meets all the requirements. How can I ensure it uses the better graphic card, I'm a mac user so I have no idea how to use windows.
If it is not a hardware question, and a Sims 3 game technical question, please head on over to the Game Help section. Links can be found under the Help menu and on every header of each wiki page for the Sims 3.
There's is also a stickied thread, with Wiki link that answer this question on there. Here, for your convenience Game Help:Intermittent Freezeswiki.
If those suggestions don't work then, please post in the Sims 3 OS X Help forum with all the things that you've tried and what happened if they don't work, and what world you're playing in.