Big house - how do you play ?

I don't play big houses for exactly the reasons you stated. It takes too long for sims to move from one room to another, especially if you build them like regular houses. You need to add more doors and alternate routes so they can travel easily from one place to another. You can't have just one tiny bathroom in a large house with many sims, you need to add more, so that they are easy to reach, and it's difficult to keep track of sims on different levels and opposite corners of the property.