As it is a bit unclear from the changelog. Will the improvements (from which i would say that some would be essential stuff) which are stated as free in beta kept free, or already hidden behind a paywall with the stable Release? At least im not really fond of the idea too test Features which should be payed later for (that is something internal tester do, not the public)
First those low speeds might be due to the Hardware as that must be able to reach the Max read/write speeds too. Second if the sd card only gets that slow speeds and the Hardware can be ruled out as a cause..... could it be that the sd card is damaged? I would test it with a limited amount of stuff on the sd card (Maybe around 20gb) to see how Poweramp behaves and to recheck if it is caused by the app or the card put the same files afterwards onto the internal save of the phone. If that works better then you at least know what went wrong. BTW. Recently switched phones and put around 151 folders with a bit over 2200 files on it (approx 80gb) and scanning took almost no time.