Considering the performance issues the Sims 3 had on even some high end PCs, especially when you had a large world with a ton of sims, performance being an issue just wouldn't surprise me. Now maybe that just has to do with it being poorly developed but I guess I'm just trying to give them some benefit of the doubt. Though I guess after Sim City and the recent news about the Sims 4 I probably shouldn't :/
Well, I'll be fair here. The Sims 3 is, on a technical level, really quite complex. The game is attempting to run AI algorithms for 100+ sims simultaneously, complete with their own jobs, relationships, daily routines, etc. Yes, the vanilla AI is horrible at this, but there are many mods that fix this. The engine is perfectly capable of doing it, and the mods don't cripple the performance or anything. Not to mention the hundreds of sim animations that need to be loaded and ready to go at all times. There's also the detailed lots and houses that are entirely customizable, down to each texture, and every object added to the houses, big and small, is a big chunk of additional polygons that need to be rendered. There's
tons of things going on under the hood.
So even on a basic level the engine is very complex. The main problem for Maxis is that they're churning out DLC and features faster than they can fix them. Expansion packs add quite a bit of new content every time a new one is released, and there's simply not enough time to actually do proper QA to resolve all the technical issues.
What Maxis is doing with Sims 4 is turning down the computational complexity, or at least attempting to compartmentalize it. But the issue here is that, at the end of the day, they're making bizarre design decisions that's giving players a much more limited experience for the sake of performance, and once the endless parade of Stuff Packs and Expansions starts coming in there's going to be unoptimized features and a big ol' mess of bugs introduced anyway. They've never been able to make a completely stable Sims + Every expansion package simply because of how fast they're churning out content.
I guess what I'm saying is, they're going through big lengths to cut down the game's complexity for better performance, but I'm not confident that they'll be able to maintain a lean, polished experience anyway, so there's no real point in simplifying the game.