Hello Chopsticks,
The way the game calculates the path of the Sims / manages their moves across a world is getting more and more complicated with every EP/EP patch. The result is that the code is more and more sensitive to any mistakes made in the routing of a world. This is true for custom made worlds, because some creators spend less time others polishing their work - sometimes just ignoring how much trouble a tiny routing mistake can cause. The very bad news is that it's true for EA made worlds too.
China is notorious for having really big mistakes, and many players have experienced trouble in that world for a long time. But Generations has made it even worst for all the worlds, including Sunset Valley, Twinbrook and Bridgeport. The changes are probably just in the patch (and all the ones that'll come after) and not only the EP itself.
Routing calculation is heavily relying on your CPU. Whenever the game is have trouble finding the path for a Sim, the game lags. Older and lower end computers suffer more than others. Turn off any process that is not needed when you play the game, play offline, play slowly indeed, be light with additional content... Anything that can relieve your computer. And for custom world, be very picky with them :-)
For the challenge, avoid like the plague the area around the Hot Springs in China and anything behind it. It's really awful out there. Try to play on speed 1 as much as possible. If the lag is unbearable, use resetsim *, this will do a resetsim on all the Sims in the world, including yours. They'll be sent home, thus unstuck from whatever place they couldn't escape.
