@Anonymous Yeah, I'm sure it's super hard to track down, because there can be so many possible causes. I went on my inventory and object deletion kick after seeing SimGuruPopcorn suggest it in a livestream, and it does seem to help, but there also might be something else up. It really does seem to be this specific lot.
(Er. We had THOUSANDS of Death Flowers in one Sim's inventory. I'm trying to clobber any item that says 99+ in addition to general tossing.)
Also, people use the word "lag" to mean different things which would also have different causes.
They were so zippy on Batuu, but they didn't have their family portraits (I'll cry if those are the issue and have considered downloading Sims 4 Studio to try to turn them into CC), their large garden, the city hubhub (they live in Torendi Tower, but it's a very small building with a very large garden by MsGryphi and I probably need to delete more decorations in favor of those beloved family portraits; I already reduced the number of plants), etc.
I will say... my Sims have maxed photography and every time someone moves out we take beautiful portraits of them first, and... I put a stack of like 30 portraits into a chest to get them out of personal inventory, and dragging them was as slow and "gluey" as dragging 100 Death Flowers in a stack. So I do, with a heavy heart, wonder if they're an issue. We have 18 displayed around the house, and 6 paintings from reference. Just in case, I did make copies and put them into the Plumbob Pictures museum, as most of them are Global Superstars. LOL. And I suppose we don't HAVE do display our "FOR THE HOARD" accomplishment of every single collection plaque? sniff... but I'd rather wander around and delete clutter I don't care about and keep my family portraits!