It is supposed to start on the day that you downloaded the update along with heat and lag. You should expect that for 3-5 days to index the files and for photo optimization using object detection algorithms.
About Apple software updates - Apple Support
Rarely does the indexing last longer, but if you find that to be the case for you, then you have 4 options:
1-- Post the screenshot of Settings > Battery, that shows the green battery graph with the last 24 hour option and the top apps consuming the battery.
2-- Perform a Force Restart:
- Press and quickly release Volume up
- Press and quickly release Volume down
- Press and HOLD side button until the Apple logo appears. This may take up to 15 seconds while holding the side button and you must ignore anything else on the screen during that time. If you do not see the Apple logo, then try again as it may take a couple of attempts to get the speed and sequence of the key presses correct.
3-- Perform a Factory reset without restoring from a backup to test. I would recommend to create a backup first with a computer, then use the computer to perform the Factory Reset so the OS gets reinstalled. Compare the battery life then, and you can always restore from the computer backup.
4-- Wait for the iOS 26.0.1 release that is currently in beta and being tested. We cannot comment on the issues that have been resolved in this beta, due to the restrictions of posting the information on a public website.