For played Sim count, mine is down to about 275 or 280 ish. I had reached over 300 recently but decided I needed to whittle that down so I just went through and marked a bunch of the ones I don't really use much, unplayed.
My performance is still pretty good, though I have a pc system way above recommended specs that's fairly new. I was getting fewer Sims on community lots for a while so I think reducing the Sim count helped.
Since I have a lot of Sims (creating Sims is one of my favorite things to do) I try to keep everything else in moderation. Sim and house inventories are usually low, I use lots with good routing space and generally low on clutter for most, I try to use the bb.moveobjects cheat as sparingly as I can, I regularly clean out my screenshot folder, delete cache files at every patch and download, and minimize my mod use. MC Command is my only script mod, and I use no autonomous phone, no celebrity walk, and no situation outfits in the magic realm. I have no CC (though I really want to use some, it's just that CAS is so cluttered already.) I also go through the unplayed households and delete most random generated townies regularly.
I play with aging off, environmental eco effects off, naps off, life balance off, Ailments off and neighborhood stories off. I keep weather effects on, romance on, invites on, and all Occult related things on. I do play with cheats active most of the time. I also use the "Save As" feature once in a while and game repair, to refresh my game save. I mainly play one major ongoing save file, so I recently moved all my Sims to a new save file and had to replace all the lots and everything, which took a while.
I do not have For Rent, High School Years, Businesses and Hobbies, or Adventure Awaits, nor Parenthood, Dream Home Deco, Home Chef stuff pack, or several kits and I feel that my game runs pretty well, no major errors or crashes, but I do have some bugs. The one thing I probably really need to clean through is my library. I have way too many things saved there.
This game, as old as it is and with so many moving parts, does require a certain level of maintenance and balance or it gets overwhelmed. That's just how The Sims is though, it's always been this way.
