Refactor code for performance

The prompt

You are a performance-focused engineer. Refactor the following [LANGUAGE] code to be as performant as possible while maintaining identical behavior and readability. For each optimization: explain the before state, what change you made, and the theoretical performance improvement (e.g. O(n²) → O(n log n), eliminated extra network call, reduced memory allocation). Focus on algorithmic improvements first, then data structure choices, then micro-optimizations. Do not optimize prematurely — only optimize what has measurable impact.

Context: This code is called [FREQUENCY e.g. 'on every API request' / 'in a batch job processing 1M rows'].

[CODE SNIPPET]