Battery

First all there will be a lot of housekeeping, re-indexing and photo curating going on after any update or after a transfer from another phone.

If you have iCloud photos turned on there will be a lot of Wi-Fi traffic.

There may be apps that are inherently incompatible with the new iOS

Start by removing any VPN or security apps. Apart from being totally unnecessary they are also a strong contender.

Many mainstream apps need an urgent update, so go to App Store and scan through your apps for those that need updating.

While there, uninstall any apps you haven't used for some time. They are unnecessary update burden. 

Check the battery usage chart in battery settings for any obvious bad apps but not every background process is shown.

Any streaming apps or apps that can run with phone closed, or Clocks that hold the phone unlocked, always force close them afterwards, despite the accepted wisdom not to.

Check Settings, Privacy, Location services and make sure none of them say Always. 

Check battery capacity is above 80% in Battery “health”. 

Only after all that do you need to do the simple stuff advised in this article:

iPhone Battery and Performance - Apple Support

In extreme cases you can backup and restore to factory.

 NB as this phone is not new you should check the battery for

Self-Discharge -

User Tip;

Check iphone battery for self discharge