There are current issues with the iPhone 6/6S under iOS 11.
You can try some of these tips to see if any or all of this helps.
How much free data storage space is left on your iPhone 6?
There should be, at the very minimum, 2.5 to 3 GBs of free data storage space, or greater!
Try a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
In Settings App under General, Reset, try Reset All Settings.
In Settings App under General, turn OFF Background App Refresh.
In Settings App under General, Accessibilty, try turning ON Reduce Motion.
Make sure you do not have scads and scads of Apps sitting in the background app switcher!
Good Luck to You!