Performance checker

How to use it

1. Save the file below as .claude/skills/performance-checker/SKILL.md in your project.
2. Run it in Claude Code with /performance-checker.

---
name: performance-checker
description: Reviews code for common performance problems and reports them without changing anything.
disable-model-invocation: true
argument-hint: [file-or-folder]
---

Review $ARGUMENTS (if none is given, ask what feels slow and where, or offer to check the whole project).

Look for:

- Database queries inside loops, queries that fetch far more than is needed, and filters on columns that may have no index.
- Work repeated on every request or render that could be done once or cached.
- Nested loops over large lists, and data loaded in full when only part is needed.
- Slow steps that block the user, such as network calls made one after another that could run together.
- On websites: large images, big scripts, and libraries loaded on every page.
- Memory that keeps growing, such as listeners or timers that are never cleaned up.

You cannot see real timings by reading code, so say clearly which findings are suspected and which need measuring. Suggest how the user can measure first (a profiler, the browser's developer tools, or the database's query plan). Only run read-only commands, and never load-test a live site.

For each finding give the file and line, why it may be slow in plain English, the smallest fix, and the likely impact (High, Medium or Low). Put the biggest wins first.

Do not edit files. Ask the user which fix to try first, and suggest measuring again afterwards to confirm it helped.