I was having a lot of save problems until I deleted a lot of things from the lot. I got rid of the two fish walls and a lot of unnecessary stuff. It did seem to help. Now the only things I'm adding are the Gallery items, and whenever I do add anything I find something to delete. The game still takes a really long time to load and shut down, but at least I haven't had problems saving it.
Good idea, I've already tried to limit the number of items in the house but could probably do that even more and I just noticed that my sims have way too much in their inventories, especially vegetables and besides life fruit it's not really necessary to have garden.
My main problem is not the long loading time but that my game crashes a lot and it seems to happen even faster lately. Also I have almost no trouble when travelling so I suspect something either Sunset Valley or just the home lot that causes it.
@Pam: Thanks for the link but I have already done all those general cleanup steps and always delete the cache files now. Most of the problems seem to be with this challenges saves and not the game in general.