Sitespeed.io is a mature, open-source web performance testing and monitoring tool that drives real browsers (Chrome, Firefox, Edge, Safari, including mobile) to collect a comprehensive set of performance metrics. It measures Core Web Vitals (LCP, INP, CLS, TTFB, FCP), visual metrics (Speed Index, filmstrip), HAR waterfalls, CPU long tasks, third-party requests, accessibility violations (via axe-core), and sustainability metrics (CO₂ estimates). The tool is used for one-off audits, CI/CD performance budgets, and continuous production monitoring, with results output as rich HTML reports, JSON, JUnit/TAP, and time-series data sent to Graphite/InfluxDB/Grafana. The architecture is plugin-based: a core message queue orchestrates a pipeline of plugins (setup → url → summarize → render). Built-in plugins handle browser testing (via Browsertime), HAR analysis (PageXray), best-practice advice (Coach), accessibility, sustainability, statistical comparison (Mann-Whitney U), performance budgets, and result storage (S3, GCS, SCP, Rsync). The CLI is built with yargs and supports ~600 options organized into topical modules. The tool is primarily delivered as a Docker image (multi-arch amd64/arm64) and an npm package. Key entry points are `bin/sitespeed.js` (CLI), `lib/sitespeed.js` (programmatic API), and `docker/scripts/start.sh` (Docker entrypoint). External dependencies include Browsertime, Coach Core, PageXray, axe-core, and the third-party-web dataset. The project has extensive CI/CD (GitHub Actions), a documentation site built with Eleventy, and a release workflow that publishes npm packages and Docker images with Sigstore signatures and SBOMs.