Its a con to slown down Iphone users to appearantly improve battery WITHOUT TELLING.
I had an iphone 6 and the battery was fine until i upgraded to ios 11. So it seems the battery was actually fine but the software they released was actually ment to harm older phones. This is what I believe as many of my friends started having issues with battery life. Also there were lots of articles about people complaining about their battery after upgrade.
My mum has an iphone se ,new, ios 10, no problem, my brother however, has the same phone but with new ios 11. NEW phone. He charges it twice a day. 
So does my colleague with his new iphone 7.
Improving battery life is just the most stupid excuse they could come up with.
It was Apple's intension to slown down phones, kill battery life and force them to buy the most overpriced iphone in history for £1000!
All this tacticc worked with me, they pushed me to my limits to not hit the wall with that phone being dead at 1pm from fully charged in the morning. Fortunately they didnt manage to rob me of £1000, and instead I left them, forever!
PS: do you still remember Apples latest court ruling for tax evasion? This is one greedy company second to none!