One thing about liquid concentrates, whether they have been mixed from powder, or bought in a pre-mixed bottle, is temperature. If you mix the working solution the night before, you have to spend time making sure they are at the right temperature by the time you use them. That takes longer than just mixing the water at the tap to temperature, and adding the liquid concentrate.
I also use replenished developer, which isn't diluted, so it's warmed up prior to using, so that it's at exactly my desired temperature before I use it. If I use single-shot developer, like HC-110 or Rodinal, I just pour the concentrate into 68 degree water that I get from mixing hot and cold at the tap. And while the film gets its pre-wash, and subsequent developing cycle, I warm up the stop bath and fixer to working temperature, so there I don't really lose any time.
What would you gain from mixing your working solution the night before? That's the part I don't understand.