They said during snowy escape that the limitations are the number of objects and unique swatches including walls, floors, roofs, etc. It also seems to extend not just to the house you're playing but the whole neighborhood. A house with excessive lived-in clutter will perform less well than the same house sparsely decorated. They even said when wallpapering walls, don't leave any bare because that counts as a different wallpaper and adds to the resource usage. It's definitely something to consider if you're having performance issues - the house you're in and the other houses around it.
