Well you can do the following:
* Restart in Safe Mode: This disables third-party apps to check if one is causing the issue.
* Update your software: Check for OS or app updates.
* Check battery health: Check you phone batter health and for slow phone go for a good phone optimizer app like Smart Phone Cleaner.
* Factory reset: If all else fails and the problem seems software-related.
* Visit a technician: Especially if you suspect hardware issues.