Hi,
First of all, make sure you are patched up to date (to 1.63).
Next, play in windowed mode so if the game freezes completely, you can use "force quit" to close it. It's better to crash out of the game than your whole computer.
Try manually resetting your RAM during playing. It can make a big difference in reducing freezing and crashing:
http://bluebellflora.com/the-sims-3-on-a-mac/resetting-ram-during-gameplay/
The game might have frozen because there is an unroutable or stuck sim somewhere. This happens quite a lot in Bridgeport so if that's the town you are currently playing, that might be an issue. Try this cheat. Type cntrl-shift-C and a blue strip will appear at the top of the screen. Now type testingcheatsenabled true and hit return. Open the box again with cntrl-shift-C then type resetsim * and hit return again. All sims in town will reset. See if the lagging is now gone.
If you are happy to use mods, NRaas Overwatch can help, as can ErrorTrap.
Caspin.