Sometimes it is just a bad battery
I had a similar problem with an iPhone 6.  The battery page in Settings said the battery was healthy but the phone wasn’t lasting long and was benchmarking at about half the score it should have.<p>Coconut Battery said the battery was around 65% of its design capacity.<p>After replacing the battery the phone went back to normal, benchmark scores improved.<p>It’s really scummy that the phone was downclocking itself to lighten voltage but not informing me.   This was all after the iPhone 6 battery scandal and software updates to show battery health.