iPhone 6S very slow after upgrading to iOS 11
The next best thing is to restore the iPhone software. Meaning erase the software and then using iTunes on a computer to restore the software with a refreshed iOS 11.0.2.


Backup iPhone via iCloud (optional but highly recommended)
Plug in iPhone to iTunes and click restore iPhone. iTunes should also automatically backup the iPhone also. - https://support.apple.com/en-ph/HT201252
**Make sure iTunes is up to date and then restore.
Once restore is done you are going to get the hello screen on the iPhone. Set up the iPhone properly then when you get the Apps &amp; Data screen, restore from iCloud backup (unless you chose not to back it up). - https://support.apple.com/en-ph/HT204184
Then your phone should be back up and working with your stuff back on it and the software cleaned up.


Let me know if you have any concerns/questions/issues.