iPhone 6s Plus battery drain with Siri and Camera

[iMore Question]

So I just bought a Second Hand iPhone 6s Plus, it was all good but when I use the camera app, it will drain very fast in the first 1-2 minutes of using (6%), after that then the battery drain stops, and in the end a 10 minutes 1080p 60p will result in a 10% battery drain(for me its normal) however the issue is whenever I opened the camera app it will drain like hell and I use snapchat a lot, opening and closing the app regularly and the battery will drain.

This issue also happened when I use Siri, just asking 3 question, it will drain 3 %.

Also the camera app is funny, it focuses slowly(not instant but gradually), and if for example I tap an area to focus, it will focus but if I tap the same area twice it won't refocus anymore and just stay still. is this normal because my friends iPhone 6s if I tap the same area twice it will try to refocus.

Please Help

 

[RECON1]

The camera app and probably Snapchat will drain some battery because of location services. You can disable that though. Unfortunately I'm not too sure about the focusing issue. Since you did buy this phone second hand, how long did the previous owner have it? One possibility could be that the battery might need to be changed. You can also try to do a full restore from iTunes.