I've looked all over an haven't found a solution to this problem. I started started the Sims 3 a few weeks ago and loading a Bridgeport save. To my surprise, the game began to lag like crazy. I'd click on a chair to tell a sim to sit, the pie menu wouldn't cum up for a couple of seconds. When the sims would walk, it lagged too. My computer has 8gb of ram and 512mb of video memory, so I know specs aren't a problem. I haven't added new custom content for at least a month, and since this is a nerw problem custom content has nothing to do with it.

Last Sunday I finally figured out what the problem was. Every sim in the town was swarming to Blumbob Pictures Backlot (owned by my active family and renamed to Wonzy Pictures Backlot). No matter what I do the sims keep coming back. I used "resetsim *" and they com back. I even bulldozed the lot and put a new backlot on it, and everyone came back. It would stop lagging for a few minutes until all the sims got back to the lot. Tawaain's overclock does not help at all. I have another Bridgeport save and it doesn't have this problem.

Screenshot: (It gets much worse the longer I play)

It only happens to one save, not both? Does it occur if you start a new Bridgeport game?
When you replaced the lot, did you demolish the entire lot or just the building?
Since you mention using Twallans Overwatch mod, do you also use his Master Controller mod?
Master Controller has a "Reset All" option that resets all lots, objects, and sims.

What's in that thought bubble that they all seem to have?

Did you try rubbing the pie menu? Be gentle.

The second save game I have in Bridgeport is one I started to see if the problem occured again. It didn't. I just demolished the building, I'll try the entire lot in a few minutes. I did try that option in Master Controller and it did not help. Awesomemod's "fixall" option was of no use either.

Edit: THe though bubles are all different, what ever that sim happens to be thinking at the time. I tried bulldozing the actual lot and replacing it with a lot of the same size, and no one seems to be swarming yet. I'll edit my post again if they come back. THanks for the help.