I hope this helps. First check how much available storage you have right now. If you see that your under 1GB then try to delete apps that you don’t use as much. Make sure that you close your background apps as well, thus making sure the brightness is not too bright unless needed so. Make sure your device is up to date on OS updates . If nothing of this is working. Then backup all of data on your laptop or iCloud (i usually back it up on both just to be safe) and confirm your data is back up, then perform a Factory Reset, on Settings - General -Reset or on the computer. I personally prefer doing it on a computer but you can pick your preference. If nothing works, then something could be wrong with your device, getting a replacement would be the choice if possible.