Why does my battery drain fast even after replacing the battery?

My iPhone 6S has been draining very fast the past few months so I decided to buy a battery off of eBay (good seller, good ratings) and replace it. There were no hiccups in the repair process, no cables were damaged. When I plugged it in and charged it to 100% today, it still drained very fast with minimal usage. I don’t pull my phone out at school at all and I started the day out with 100%. After 7 hours my phone was down to 3%. Keep in mind I don’t pull my phone out a lot in school. In the battery tab of my settings, the apps look normal with my social media apps taking ~25%. However, when I see the time stamps, there are only about 30 minutes of use on them(active and background). I installed Battery Life Doctor on my phone to monitor the battery health and it says the new battery is fully healthy at 1715 mAh. On the estimate for how much time I have left until my battery runs out, it says a little less than 10 hours, but my battery decays very fast. It will jump from 7:30 remaining to 7:10 remaining in a matter of a few seconds. How do I troubleshoot this?