I previously installed insider build .71, I then unsubscribed from the program to install the firmware update: I think that it's the first time I see a significant step forward on the phone. I unplugged the phone from the charger at 7:30 AM, now it's 10:45 PM and I still have 30% battery. I had data connection turned on all day long, together with some WhatsApp, Viber and Outlook usage.
I will have to test it better in the upcoming days, but I hope this is a good sign while waiting for further updates and improvements.