How do you use your iPhone?
Use is a factor, too!
Some users work their iPhone harder than others and that can have an effect on speed/performance of any iPhone due to limited RAM and CPU resources.
Have you tried 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?
How much free data storage space is actually left on your iPhone.
iDevices need to maintain a minimum of 3 GBs, OR greater, free data storage space.
In Settings app, General settings panel, turn OFF Background App Refresh.
Settings app, under General settings, Reset panel, at the right bottom of the list, Reset All Settings.
In Settings app (NOT from the iOS Control Panel) turn OFF Bluetooth when not using any Bluetooth devices.
Also, in general, if you want a faster IDevice, 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!