Apple regularly provides software updates that improve your device with new features, important security enhancements, and bug fixes. Learn more about why they are important and how they work.
support.apple.com
“New features are exciting and help you get even more out of your Apple product, though some may require additional resources from the device. Depending on individual usage, some users may notice a small impact on performance and/or battery life. Apple continually works to optimize these features in software updates to ensure great battery life and a smooth user experience.”
Even Apple admits it. And this statement is clearly separate from them saying there will be battery drain while the update is completing in the background. But let’s stay on topic.