Apple delayed plans to roll out CSAM scanning, so that is not included in iOS 15. You are partially right though, in that the likely explanation for this extra

Apple delayed plans to roll out CSAM scanning, so that is not included in iOS 15. You are partially right though, in that the likely explanation for this extra power consumption is re-indexing content / rebuilding local neural networks, which use a lot of power (for a day or so, and then they're finished). So rather than scanning your photos for CP, more likely it's scanning your photos for text or similar as part of the new Live Text feature.