I'm going to be a bit blunt here. TS3 is a mess. It's not really a game the way I see it, none in the sims series are, it's a way of life platform for creativity, design, and storytelling but the platform is cracked in many places. From a programming standpoint, it's a horrible mass of spaghetti code slapped together and expanded upon by various disjointed development teams in such a way that a bunch of five year olds with attention deficit disorders could have done a better job.
That doesn't mean many of us don't love it, have a lot of fun working with it, and have much respect for the often berated over-extended game engine and many of the things EA did with it. But it does explain why we have to give it such a strong environment on which to run compared to what it "should" take in order to get decent performance on longer running ongoing games and especially with all or nearly all EPs in play.
I can't say exactly why CASt for example runs so much better for you on a WA world than your homeworlds except to point out that the WA worlds are typically lightly populated by comparison and even with mods we don't usually progress the resident population there through their lives and their careers (if they really have any beyond the marketplace vendors) as we do elsewhere, so the game engine does have much less to do on those vacations even when things are temporarily standing still for a design session.
