Just to trow my 2 cents:
- sell the paintings, sculptures, photos, collectables, books, relics, everything you don't really need for dynasty
- keep the number of objects on lot on minimum - no need for 8 or 9 bedrooms, 10 counters, countless decorations around the house, or dozens of roses outside - if you aim for minimum load time.
- I'm not certain about this one, but I believe it contribute to the loading time - more objects you recolor - longer it will take to load.
- As Saltypaws already said, large garden, sprinkles and fountains, ponds, and detailed landscape - can make it longer to load
-Clean their inventories too. Don't accumulate harvestables, fish or anything really. With that fridge from culinary career and with (upgraded)food replicator you can have many many free meals that lasts forever.
In most of the cases it is either you have a house to show it off, filled with everything but takes ages to load, or you have simple, almost boring house that is fast to load.