The iPhone performance is directly tied to battery life. The more power the battery has the better performance you will get. Try changing these settings to get more power longer from your battery:
2) Background refresh: Not everything needs to be constantly refreshed. Turn off refresh on most apps. To do this: go to Settings, General and Background App Refresh.
3) Stop Fetch for your email and other apps ALL the time. Go to Settings, Accounts and Passwords, Fetch New Data and then toggle to turn off.
4) Screen Time Out: Shorten the time it takes to turn off your screen. I changed the setting from 5 minutes to 3.
5) Brightness. Does the screen need to be THAT bright? Try lowering the brightness to save power. Go to Settings, Display and Brightness.