@Jessmaadge It's possible, though unlikely, that something in your old Sims 3 user data folder was corrupted when you installed Showtime. It's also possible that the increased load was a bit too much for your computer to handle—your specs are somewhat low for the packs you have installed. I'm not saying it's definitely a problem now, especially if you'd been playing fine before, but Pets and Seasons might be too much for your hardware.
What I would guess though, not that it can be proven, is that when you installed Showtime, the game spawned a few street performers, as it always does if Showtime is enabled, and then those street performers, or the people watching them, became stuck. Most of the issues with the world of Isla Paradiso have to do with bad routing: a sim gets stuck somewhere and can't move, and then the game spends all its resources trying to reroute the sim, which never works. Meanwhile, everything else grinds to a halt.
When this happens in general, you can temporarily fix the issue by bringing up the cheats console (ctrl-shift-C) and entering "resetsim *" without quotes. The asterisk will instruct the game to reset all sims. That's pretty disruptive to your own sims' lives though, and in IP at least, the problem usually comes back soon enough. That's why playing in a fixed world is so important.
The other issue with Showtime is that it brings the career performance limo. Unlike other vehicles, these can spawn by the hundreds or even thousands in a single day and drag down the game's overall performance by a considerable amount. It's worse in Starlight Shores, with all the sims already high in their careers, but it can theoretically happen anywhere.
If you use or are willing to use mods, the best solution to this issue is to install NRaas Traffic. It shuts down the spawning of the limos by default, as opposed to NRaas Overwatch, which cleans them up once a night. (I've heard of Overwatch removing as many as 50,000 vehicles during its nightly cleanup.) Other than that, you might need to not play in Starlight Shores, or else change sims' careers around, although not everyone has a problem with the career limos.
As for your current save, I hope fixing the world yourself will help enough. Again, if you're interested in mods, NRaas ErrorTrap, Overwatch, and MasterController would be very helpful in keeping the world running as smoothly as possible, although you'd still need to fix IP. But the mods will help clean up errors and reset stuck sims, plus MC gives you the option to reset anything from a broken object to the whole town at once if you want.
P.S. Your dxdiag lists a few errors with Windows Defender and McAfee. They might be conflicting; it's best to shut down Defender if you're using something else instead.
