@indycutie Overloaded personal inventories can certainly impact game performance. I try to keep my sims' inventories reasonably close to empty and dump everything in the fridge or in World Adventures treasure chests. There might be a delay in opening the fridge or one of the chests, but much better that than constant lag. It's less common for the household inventory to be the issue though.
Still, you could try selling everything (and I do mean everything) in all inventories and reloading the save to see whether it helps. Use "save as" and rename the save so you can revert to the original after the test. If performance is better, you can go back and be more selective in what you sell.
Without mods, Sims 3 will generate immigrants for pretty much no reason at all. The main mod that stops this immigration is NRaas StoryProgression, but as mentioned, it's a heavy mod and might make things worse overall. I think there might be a standalone mod that stops immigration, but I can't remember exactly where I read that. If you're interested and can't find one, let me know, and I'll look around.
In the interim, the easiest way to stop sims from moving into a house is to remove the fridge. This will stop resident sims from moving in as well, but you could manually move families around in Edit Town instead, putting back the fridge immediately beforehand.