Hi!
First of all, make sure that the phone is up to date. If this is the case, but it's still very slow. Then restore the settings first without removing all data, sometimes this help. Do you also have a virus scanner on it? If you've tried everything, then the last resort is to completely format the phone and start over again (make sure all your data is ***backed up***!!, because if you format the phone, you will lose all your apps and data!!). After this, restore your backed up data again.
Good luck!