Things you can do to preserve battery life.
If you can, keep the screen brightness down.
In Settings app, General settings panel, turn OFF Background App Refresh for any apps that you feel do not need background data refreshing OR turn OFF Background App Refresh entirely/globally for the entire device.
In Settings App, under Mail settings, set the frequency of checking/fetching email to a longer fetch duration or to manually fetch email when the Mail app is launched.
Also, in general, if you want a faster IDevice, and to further preserve the battery, on all of my iDevices, I turn off most of the iOS motion graphics eye candy, by simply turning ON Reduce Motion in Settings app, General, Accessibilty settings.
Make sure you aren't running scads and scads of background apps in the iOS 11 Control Panel/App Switcher.
If you are, you need to quit the bulk of these background running apps by tap and hold a finger on an app window in the switcher and slide your finger upwards to quit an app. You should be able to use more than one finger to quit more than one app window simultaneously.
Also, make sure you don’t have scads and scads of active website tabs running in the web browser.
If you do, greatly reduce the amount of active website tabs your web browser.
Good Luck to You!
