A performance-testing checklist for web pages, builds, and database work that requires measurements before and after a change. It covers loading speed, page rendering, bundle size, database queries, caching, and memory leaks.

Use it when investigating or optimizing a performance problem. It measures a production-like build in a real browser, repeats browser measurements, and compares the results.

It replaces guesses about speed with measured evidence. It helps show whether a change actually improved performance and whether it introduced retained memory.

perf-checks — Skill for Claude Code by jhlee0409

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add jhlee0409/omni-harness-kit --skill perf-checks

Clone the repo

git clone --depth 1 https://github.com/jhlee0409/omni-harness-kit

Made for: Claude Code.

Or install harness-kit, the plugin that ships this one along with the rest of its 18 skills, 28 agents, 2 hooks.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/jhlee0409/omni-harness-kit/perf-checks/github.svg)](https://agentmods.dev/skills/jhlee0409/omni-harness-kit/perf-checks)

Your own site

<a href="https://agentmods.dev/skills/jhlee0409/omni-harness-kit/perf-checks"><img src="https://agentmods.dev/badge/skills/jhlee0409/omni-harness-kit/perf-checks/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/jhlee0409/omni-harness-kit/perf-checks"><img src="https://agentmods.dev/badge/skills/jhlee0409/omni-harness-kit/perf-checks.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 144 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,706 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00144	$0.01706
Opus 5	$0.00072	$0.00853
Sonnet 5	$0.00029	$0.00341
Haiku 4.5	$0.00014	$0.00171

Measured 10d ago against content hash a18b4511d8b8, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

perf-checks scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

adapters/omp/skills/perf-checks/SKILL.md · 60 lines

How it starts

The opening of the file, as written. The whole thing — 60 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Performance — measure-before-and-after methodology (omp-native)

A performance change is NOT done until a before number and an after number prove it. Prediction ("this should be faster") is not evidence. This skill runs on omp builtins only: browser (puppeteer page/tab, incl. CDP session), bash, lsp. No external MCP.

Iron rule: every claim = a measured delta. LCP 3.9s → 1.6s (-59%), bundle 842KB → 410KB, retained heap +38MB over 3 iterations → +0.4MB. A verdict with no number = INVALID.

Setup

Start the dev/prod-preview server on its own port; wait for a stable anchor before measuring. Measure a production build where possible — dev bundles distort LCP/bundle numbers.
browser open the target route. Warm once (discard first load), then measure — cold-compile noise is not the signal.
Take 3 runs of any browser metric; report the median, not a lucky min.

The ordered checks (measure at each; record the number)

Core Web Vitals — in the live page via tab.evaluate, read real entries, not guesses:
- LCP: new PerformanceObserver on {type:'largest-contentful-paint',buffered:true} → last entry .renderTime||.loadTime and .element.
- CLS: observe layout-shift, sum value where !hadRecentInput.
- INP/interaction: observe event/first-input durations; or performance.getEntriesByType('event').
- Paint: performance.getEntriesByType('paint') (FP/FCP). Navigation timing: getEntriesByType('navigation')[0]. Record: LCP _ms, CLS _, INP _ms, FCP _ms. Targets: LCP <2.5s, CLS <0.1, INP <200ms.
LCP breakdown — split the LCP into TTFB / resource-load / render-delay from navigation + the LCP element's resource entry (responseStart, requestStart, responseEnd). Attribute the dominant slice, then fix ONLY it: TTFB→server/edge; resource-load→preload/priority hints, correctly-sized/webp image, no lazy-load on the LCP image; render-delay→critical CSS inline, unblock render-blocking JS/font. Re-measure; cite render-delay 2.1s → 0.5s.

Read the full file on GitHub · 60 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

10d ago First seen · 60 lines · 144 tokens per session scan A a18b4511d8b8

Subscribe to this mod's changes

perf-checks is a skill published in the GitHub repository jhlee0409/omni-harness-kit (2 stars, last pushed 1mo ago), licensed MIT. It adds 144 tokens to every session and 1,706 once invoked, about $0.0007 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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perf-checks

Getting it into your agent

Wrote this? Show the measurements

What it costs to keep this loaded

Grade A, and why

Nothing flagged

How it starts

Performance — measure-before-and-after methodology (omp-native)

Setup

The ordered checks (measure at each; record the number)

What this file has done since we first saw it

Other skills, from other repositories

journey-simulation

caliber-testing

test-run

slowest-tests

symfony:e2e-panther-playwright

symfony:tdd-with-phpunit