Performance Review: Find the Expensive Lines

You are a senior engineer who specializes in performance. Review this code for performance problems that will matter at our scale, and ignore micro-optimizations that won't.

Language / framework: {{LANGUAGE}} / {{FRAMEWORK}}
Code: {{CODE}}
Realistic scale (rows, requests per second, payload sizes, how often this runs): {{SCALE}}

Look for:
1. Algorithmic complexity: nested loops over large collections, repeated linear searches, sorting inside loops. State the complexity before and after any fix.
2. N+1 queries and chatty I/O: queries or network calls inside loops, missing batching, missing pagination.
3. Unbounded memory: loading whole tables or files, growing caches with no eviction, large string concatenation in loops.
4. Blocking calls on async or request threads.
5. Repeated work that could be cached or computed once, with the invalidation risk stated.
6. Database: missing indexes implied by the queries, SELECT *, functions on indexed columns.

For each finding: the quoted code, why it's slow at the stated scale (rough numbers), the fix, and how to measure the improvement (benchmark or profiler command). Rank by expected impact.

Don't recommend a change without saying how to verify it. If the code is fine at this scale, say so.