I still think the big mistake was removing the limit on the number of Sims you could have. A world with 30 lots is potentially room for 240 Sims (presuming you don't use mods to squeeze even more Sims into a household). They can't reinstate a cap on the number of Sims, since that would be disastrous for people with overpopulated games (Sims would either disappear or you'd have to delete some in order to add more kids).
I've noticed that lag in my game primarily occurs during the day, when there are random Sims wandering around the neighbourhood. You can only load one neighbourhood, but things are going on all the time with Sims in other worlds - they die of old age, visit your neighbourhood, and call and text the Sims you're playing incessantly. Sims who aren't even in a world appear in the game, and even the dead Sims don't die, but become ghosts. (I think this might also be the reason we aren't likely to get apartments in other worlds - again, that allows for even more Sims in the game, without adding extra lots.)