I'm not sure when it started, but my phone is draining its battery even when being charged. It's likely to be caused by some apps I installed recently. How to find out which apps are causing the problem? Is the builtin
Settings -> Battery statistics accurate enough?
I think the "Battery" in "Settings" might give a hint and you should turn off those apps working in the background when you don't use them.
And play your phone in somewhere cool, because hot atmosphere will also drain the battery fast.
Also you can download a third party app like GSam Battery Monitor(https://play.google.com/store/apps/details?id=com.gsamlabs.bbm) to check out what makes your battery drain so fast.
well, to be honest if you are rooted, you should install ampre, and measure if you give your phone enough juice. if you drain battery faster than charge, even if screen is off, or if you have good charger and quality cable (good charger means like 1.5Amps and higher) you have broken USB port. its very common, and it makes impression of super slow charging or discharging while beeing plugged in.
I hope it will help you.