I’ve found this kind of lousy battery life on my iPhone 6s with iOS 12.4.x. I have been appalled by the battery life on this phone from day one. My prior phone was an iPhone 4, which still lives on happily as an iPod now.On both of my 6s’s, since upgrading to all versions of iOS 13 when ever I use the phone it blows through the battery.
Since iOS 13.3, if the phone is idle, the battery lasts like normal. But if I actually use the phone it blows through the batter in only an hour or two of constant usage.
Already had the batteries replaced in both. One was completely replaced (they broke it changing the battery). Other just had a batter swap (both at Apple store). Didn’t help.
Wife finally gave up and bought a new 11pro (she’s very happy with it). Battery lasts all day with heavy usage. Better than my XSM.
Seems like iOS 13 is running the 6s CPU harder either to keep up with the boatware or the kernel is less optimized for the older CPUs.
Honestly, looking back, you’re probably better to stay with 12.