---
title: "Measuring it yourself · Agent Wranglers"
url: https://agentwranglers.se/playbook/website/measuring-it-yourself
updated: 2026-10-20
licence: "All rights reserved"
---

# Measuring it yourself

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

> A repeatable lab for website features. It runs Lighthouse with and without each one, a Playwright click test with added latency, and live checks from outside.

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers website playbook: https://agentwranglers.se/playbook/website/measuring-it-yourself`.

## The method

1. Copy the repository once, so edits made during the run can't leak into it.
2. For each experiment, copy it again. Apply one small edit script that removes a feature, or adds one back.
3. Build the copy and start the server inside a container. Run Lighthouse 3 times on two pages, phone and desktop, and keep the median. One run on one page is enough for SEO and accessibility scores.
4. Lighthouse can't see clicks, prefetch, the service worker or repeat visits. For those, run a Playwright script in 7 fresh browsers per variant. Add 100 ms of round trip and a 20 Mbit/s limit.
5. Compare each variant with the baseline built the same way.

## Lab image

```sh
cat > Dockerfile <<'EOF'
FROM mcr.microsoft.com/playwright:v1.57.0-noble
RUN apt-get -qq update && apt-get -qq install -y iproute2 curl >/dev/null
WORKDIR /lab
RUN npm init -y >/dev/null && npm i --silent playwright@1.57.0 lighthouse@12.8.2 \
 && ln -s /ms-playwright/chromium-*/chrome-linux*/chrome /usr/local/bin/chrome
ENV CHROME_PATH=/usr/local/bin/chrome
EOF
docker build -t web-lab .
```

Give the container `--shm-size=1g`, or Chrome's tab crashes at desktop size. To add delay, also give it `--cap-add NET_ADMIN` and run `tc qdisc add dev lo root netem delay 50ms rate 20mbit`. That is 50 ms each way on the loopback device, so a 100 ms round trip.

## Lighthouse runner

```js
import lighthouse from 'lighthouse';
import * as chromeLauncher from 'chrome-launcher';
import desktop from 'lighthouse/core/config/desktop-config.js';
const [,, base, runs = '3', ...pages] = process.argv;
const med = a => [...a].sort((x, y) => x - y)[Math.floor(a.length / 2)];
for (const form of ['mobile', 'desktop']) for (const p of pages) {
  const rs = [];
  for (let i = 0; i < +runs; i++) {
    const chrome = await chromeLauncher.launch({ chromeFlags: ['--headless=new', '--no-sandbox', '--disable-dev-shm-usage'] });
    const { lhr } = await lighthouse(base + p, { port: chrome.port, output: 'json' }, form === 'desktop' ? desktop : undefined);
    await chrome.kill();
    rs.push({ perf: lhr.categories.performance.score * 100, lcp: lhr.audits['largest-contentful-paint'].numericValue, cls: lhr.audits['cumulative-layout-shift'].numericValue });
  }
  console.log(form, p, Object.fromEntries(Object.keys(rs[0]).map(k => [k, med(rs.map(r => r[k]))])));
}
```

## Click and repeat-visit test

In each fresh browser, load the start page. Record HTML arrival, FCP and LCP from `connectStart`. Wait 2.5 s, so prefetch and the service worker can warm up. Click a link and record `performance.timeOrigin + FCP − click time`. Then open the start page again in a new tab of the same browser. Record its FCP as the repeat visit.

## Useful tricks

- **Prefetch did not fire?** In the Chrome DevTools Protocol, call `Preload.enable` and log `Preload.prefetchStatusUpdated` events.
- **Does a resource hold up paint?** Abort it with `page.route(...)` and compare.
- **Test the TCP start window on its own.** Run `ip route change $(ip route show default | sed "s/ initcwnd.*//") initcwnd 10 initrwnd 10`, then restore it.
- **Check production from outside.** Use `curl -w '%{http_version}'` for the HTTP version. `curl --tlsv1.1 --tls-max 1.1 --ciphers DEFAULT@SECLEVEL=0 -v` must fail with the server's `protocol version` alert. Without the cipher option, OpenSSL 3 refuses TLS 1.1 itself, so the test proves nothing. Look up AAAA, CAA, DS and `_agents` records with DNS over HTTPS by querying `https://cloudflare-dns.com/dns-query?name=…&type=CAA` with `accept: application/dns-json`.

- Previous: [Measured results](https://agentwranglers.se/playbook/website/measured-results)
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