First off, Sims 3 is a huge game. There are many, many things that need to be load into the game at once
, such as each sim, with its own traits, personalities, etc., objects, the lots, the things inside them and so on and so forth.
The longer you keep a game going, and the more things you have into a certain save, the more information the game will have to load
each time, so you should be patient from the get-go.
That being said, you might be experiencing an issue that you shouldn`t have to, but first you should check the system requirements. I recommend you visit canyourunit.com and select the game from the list to see if you`re computer meets the minimum requirements indeed. Even if it does, how much wiggling room does your computer have? Is it barely meeting them, or are you way over the minimum? This should also tell you quite a bit of how the game it`s going to perform.
More specifically to your problem, the houses that already come with the game or its expansions come pre-cached into the game itself. If you build a custom house, the game has to take note of every single thing you put inside the house to the last nook and cranny, hence taking much longer to load
than normal houses.
As a final note, even though The Sims franchise is a great series, the later titles, especially Sims 3 base and Sims 3 Late Nights are reputed to have somewhat bad coding, affecting the performance on most computers. There`s simply nothing you can do about this.