Massive battery drain after factory restore
How old is the phone? It sounds like the battery is getting on in age, which typically leads to rapid battery life loss under heavy load.
Greenify helped me squeeze extra battery life out of my phone, it may help you too. Keep doing tests on how quickly is loses charge in different apps. Try using MX Player or your web browser to play the youtube video instead of the youtube app. The only other suggestion I have for increasing battery life would be switching to a custom ROM more suited for battery life preservation. Anything else either consider replacing the battery, or getting a 2nd external backup battery/case