Calibre

Calibre is a web performance monitoring service that combines three data sources: synthetic page speed tests, Google Chrome UX Report (CrUX) data and real user monitoring (RUM) of Core Web Vitals collected from visitors’ browsers.

Synthetic tests provide lab data to find the cause of slow pages and to validate fixes. Calibre ranks the monitored sites by performance, shows which pages pass the Core Web Vitals assessment, and analyses the impact of third-party resources. It has an API and a command-line client, and integrates with deployment workflows.

Calibre is a commercial service sold by subscription, with a free trial. A free one-off Core Web Vitals report is also available on its site.