Hi Jody,
I can't promise this will help speed up your system a whole lot. But a few things that might help are:
Try medium graphics settings within the game as Saltpastillen said, Pets is pretty graphic intensive.
Try deleting the cache files every now and again (socialCache.package, Sims3Logs.xml, simCompositorCache.package, scriptCache.package, compositorCache.package, CasPartCache.package)
-- Also delete everything except Ccmerged.package file in your DCBackup folder. -- Documents > Electronic Arts > The Sims 3 > DCBackup
Restarting the computer is helpful too. Specially if you play Sims for hours (2+hrs a day or more) Saving every hour or so to prevent game-play loss.
Run registery cleaner program, like Ccleaner -
http://www.piriform.com/ccleaner/download It helps clean up temporary files, internet cache files and other files that are left behind and or that are no longer being used, after reinstalling/uninstalling.
If you are playing a big family, take a look at their personal inventories, and the family inventory as well. Get rid of things that aren't needed. As having to many items (100 or more items) in their inventories may cause lag and freezing. This can happen fast if the sims is a fisher, gardener or alchemist, not to mention collector.
If you have store content, you may want to look for the Tree of Prosperity
The link below may give other helpful tips as well.
Sims in Slow Motion
http://sims3.crinrict.com/en/2013/11/sims-in-slow-motion.htmlHope this helps
MrsFlynn