The first question I would ask is, how old is the battery? I have the previous model, the 6 Plus. Just this past December, I had a sudden marked decrease in my charging capacity and battery power, though it never died, though it did do weird things like register 38% when I turned the phone off, only to drop immediately to 33% when I turned it back on. At no time did Peak Performance Capability ever display anything other than "Your battery is currently supporting normal peak performance."Yesterday, making no calls on my iPhone 6S+, at 5pm, it was at 43%. I did a reset (hold power + home/touch button), the phone came back up and was at... 72% battery.
By 10pm, it was down to 60%. Again, no calls. I had no apps open. Just normal Bluetooth/WiFi active. Max capacity still at 91%.
Peak Performance Capacity always shows "This iPhone has experienced an unexpected shutdown because the battery was unable to deliver the necessary peak power.
Apple nonetheless diagnosed the battery as having failed and promptly replaced it. Problem solved.
With your combination of symptoms, your battery seems like a prime candidate for replacement. I really don't think any settings change you can make is likely to fix what you're experiencing.