More advanced girls level faster and more easily than earlier girls.
That is only half the truth. The other half is, the faster bit is miniscule.

At least with the coaching mechanism it is possible to change your main hitter in a reasonable time (read that as 2-4 month).

The diff between my Ice (300somethin stamina) and my Indira (800something stamina) is from 42k to 48k practise for a whole day.

I haven't seen anybody talking about 'Warp Bonus' and its importance.
That should go without saying. The problem is, it costs diamonds and warp is capped.

If you do not have many diamonds, you should not spend them on non permanent thingies.

Energy, studios and warp are permanent. A x8 warp can cost you over 200 later on. That is more than all studios combined or two energy upgrades.

The assumption is this: one is already on their personal warp capping due to diamond costs or at the real warp capping.

The discussion could be: How much energy/studios can be bought when not already at the warp capping. Without hampering progress too much. That is a difficult question. At least the cheapest studio should be okay in any case.

A case scenario: you have 200 diamonds and could buy either a x8 warp or all studios. The x8 does not bring you to the capping, there would be yet another x8 to buy after that. No matter your reset bonus, you will not complete the latest girls. So what do you do? Buying the warp and getting maybe 1-2 more girls, still missing 5 or so. Or buy all studios and level up stamina of the girls you have much, much quicker?