Hey, I think I've found a cause of lag that you haven't mentioned anywhere: Plants. Literally plants.
Let me explain: Whenever I'm in the Magnolia Blossom park, the game lags horribly. While there is somehow no lag whatsoever in the southern neighbourhood of Oasis Springs (where Zoe Patel etc. live). And the only difference between them is the amount of plants, with the park having huge trees everywhere and South-OS having just about nothing.
Also, I just downloaded an Irish Pub from the gallery. It lagged extremely. But ever since I deleted the ground plants it doesn't lag even nearly as much anymore. And my computer really isn't the best, so that's a HUGE improvement.
When I took a closer look I noticed that all of these things have animations. Especially visible on the trees. With the ground plants, the lag might also come from a high poly count and a lot of things sticking out of them (leaves, flowers). But deleting ground plants also removes some laggyness from the game.