Every time Apple releases a major iOS update, a common complaint surfaces: my iPhone's battery life is suddenly terrible. The issue is so widespread that Apple has released an official support document to explain it.

The company states that the temporary battery drain you experience is a completely normal part of the update process.

The Hidden Work Your iPhone is Doing

The primary reason for the sudden battery drop is not a flawed operating system, but rather the massive amount of work your device is doing in the background to set up the new software. This process typically takes 24 to 48 hours and includes:

• Indexing and Optimization: Your iPhone is re-indexing all your photos, files, and apps to ensure the search function works correctly. It’s also optimizing apps to run smoothly on the new operating system.
• Downloading Resources: New features and system files often require your device to download new resources in the background, consuming extra power.
• New Feature Activity: The initial setup for new, resource-intensive features can temporarily impact both performance and battery life.

Apple assures users that these processes are temporary. Once your device has completed this setup, battery performance should return to normal. While new features may have a minor, long-term impact on power consumption, Apple continuously optimizes its software to maintain excellent battery life and a fluid user experience.

Source: Apple