It happens with a lot of families, just that one lot has it really bad. It does it at parks and other houses as well and I don't know what is doing it. Could sim corruption cause low frame rate? It also does it when rotating the camera.
Not sure but stuck sims can make the best machine lag. If the lot has a lot of collectibles and stuff, that needs a lot of power too.
Try resetting all your Sims in your town with the cheat resetsim *.
Here's some more tips: http://answers.ea.com/t5/The-Sims-3/FAQ-Lags-amp-Freezes/td-p/646
