The reindexing explanation is real, it is overused, and there is a way to tell.
What actually happens after a major update. The device re-analyses a lot of content — rebuilding the search index, re-running photo analysis, and re-optimising app binaries. On a phone with a large photo library this is genuinely hours of work, and it is deliberately scheduled for when the phone is charging and idle, so it stretches over several nights rather than finishing in one go.
Why it gets overused as an excuse. Because it is unfalsifiable if you do not look. It becomes the answer to every complaint, including the ones where something is really wrong.
How to tell. Look at the battery breakdown by activity. What you want to know is whether the drain is on-screen or in the background:
- Background-heavy drain that shrinks each day is the settling-in story, and it is true.
- A single app at the top that you barely used is a real problem and it will not settle. This is the common case people mistake for reindexing.
- Screen-on drain being higher than before is not reindexing at all — that is a display or brightness change, and reindexing does not run while you are using the phone.
So look at whether one app is responsible before you accept the general explanation.