I basically ignored all the early ones except what I needed to get the funds for the later ones and got basically all the more expensive girls on level 9. Now it's taking a loooooooooooooooong time to get even 25 levels while I'm still 400 or so away from reaching level 10, as in multiple days. It's bullshit. You can't switch girls out for cheaper ones to level so you're screwed as soon as you get them meaning if you start with expensive ones you will not be reaching the next company level for a very long time, making almost no progress.