About the battery usage on your iPhone, iPad, and iPod touch - Apple Support
iApplebytes confirms iOS 13.5.1 has a worse battery life, geekbench 5 battery test is a intensive test. On my iPhone 11 i got around 9-10hours of screen on usage and on geekbench
Video: iOS 15.4.1 Battery Drain Test on iPhone 13, iPhone 12, Older iPhones
How to Check an iPad's Battery Health
iPhone Battery and Performance - Apple Support
How to check battery health and disable iPhone battery performance throttling in iOS 11.3 - 9to5Mac