How do you make your big houses playable?

I agree that having a large, old save makes things trickier. I'm not sure how relevant computer specs are, but here are more ways to combat lag (it's just hard to say how much they'll help):

• regularly repair the game
  
• use only a few floors/wallpapers and make sure that there are no unpainted rooms
  
• limit plants, mirrors, fountains, Cottage Living/Horse Ranch animals, perfectly round CC objects (high poly), fancy CC statues (high poly), detailed jewelry on Sims (high poly), chandeliers (might be high poly), and other high polygon objects
  
• clear cache
  
• clear screenshot folder
  
• have only a few saves in your Sims 4 folder
  
• limit files in your tray folder/library
  
• keep inventories (household and individual Sim) limited or empty
  

I remember that particular palace now. It's lovely. I use some of the same CC, my save is over 5 years old, and I have a similarly sized home (mine is slightly smaller) on a 64x64 lot, but I'm lag-free. I don't necessarily follow the things I've listed either. Sometimes lag seems to be a matter of luck or just comes down to computer specs. Good luck!