As you have mentioned above that your phone is damaged, this seems the obvious reason that something in the phone’s board may be dysfunctional. You should also try uninstalling some apps that you do not use frequently. Try to use apps that are from verified developers. Uninstall any games. Another reason your phone is slowing down could be because of some malware or virus. Install a good antivirus and scan your device. In none of the above mentioned methods prove to be helpful, then the last way would be to factory reset your phone. I hope this will help.