Hi Karlissa,
If you're willing to install just base game, for a test or two, here's something you can try, if you haven't already.
In the graphics options,
Check that Advanced Rendering is checked On
Make sure Animation Smoothing is checked On
Change the game's resolution to 1024x768
Set Sim Detail to Medium
I'd like to go off of the question that Steellace asks, is this lag with a new game as well? Or just older saved games? If unsure, would you be willing to try a new game to see if that may help?
It's still a possibly that the laptop overheated even with the cooling mat, due to the processor having to be in turbo mode while the game. As the processor is lower than the minimum requirements of Sims 3. Which is 2.4ghz.
My laptop even with a cooling mat, still gets pretty warm playing Sims 3 base game, due to it being a resource intense game.
We're happy to try to help.
MrsFlynn