Now I've got all of my Sims in actual houses I can resume the game.
I noticed the game had started taking ages to recalculate four times a day again.
This is the problem I had in the previous save and prompted me to move them to this new save.
The only thing I did that could have caused this is move too many Sims into the world. So I annihilated a few of them.
The Sims I got rid of are all Sims I've not trained yet and, regardless of how much I wanted them in my game, they were really just taking up space and processing power. They are now gone. They can return as part of a later generation if needed.
This seems to have solved my problem.
Annihilated Sims include: Ella and Geno Costello :'( ; Gina Genie and Geordie :'( ; My three castaways. And a few others.