Battery drains fast after being replaced for 3 weeks only

I wonder if Apple did replace a new battery for my iPhone 6s. It’s only been roughly 3 weeks. It drains so fast even I was just browsing sites without watching videos or listening to music or having other apps on while browsing. I’ve noticed it since I took my phone back aftering being replaced. (The battery level was at 9%). And I was told that they couldn’t keep it as high capacity which I understood that. However, the performance of the battery life has been being sucked out for unreasonable speed I’d say.

Like I charged my phone last night to as 100% before I went to bed. And I turned it off for a whole night then turned it on after 9am. All I have been doing are briefly checking emails, Instagram and photos without listening to music or watching videos. Up until this moment (10:30-am), it’s been on for over barely an hour using my phone. The battery level has already drained to as 70%. It’s never happened when I first bought my phone. I’ve never been a heavy user which I why my old battery level was at 83% capacity when I took it to Apple store for replacement. It’s really odd that the performance of this “new” battery has been draining so rapidly. I wonder if the battery I’ve got is really a brand new battery.