One thing I never understood about the Sims 4 is why the districts aren't open?! I mean when you are in your active lot, the game has to load the whole environment, all the NPCs, all the surrounding houses and it even loads some of the objects you can see in the houses through windows or balconies. So the game basically loads everything in the whole district yet you still need a loading screen to change lots...
The only thing missing is the inside of the houses which is a) not very demanding and b) they could make it so you can "knock on door" so that the furniture has time to load. I cannot justify the 20 second loading screen unless the problem is that super outdated engine they are using which would be understable.