Metrics Guide | Calibre Documentation

Calibre reports Core Web Vitals and dozens of other important metrics. This guide lists each metric, what it measures, and which data source it’s available in (Synthetic, Chrome User Experience Report (CrUX), and Real User Monitoring (RUM)).

Core Web Vitals#

Metric	Description	Synthetic	CrUX	RUM
Largest Contentful Paint (LCP)	When the largest image, video or text element was painted to the screen.	Yes	Yes	Yes
Cumulative Layout Shift (CLS)	The sum of scores from all of the unstable elements that move around during page load.	Yes	Yes	Yes
Interaction to Next Paint (INP)	Visual delays during user interaction.	No	Yes	Yes

Paint#

Paint-based metrics describe the visual experience of a page during page load.

Metric	Description	Synthetic	CrUX	RUM
Largest Contentful Paint (LCP)	When the largest image, video or text element was painted to the screen.	Yes	Yes	Yes
Cumulative Layout Shift (CLS)	The sum of scores from all of the unstable elements that move around during page load.	Yes	Yes	Yes
First Contentful Paint (FCP)	When the browser rendered any text, image, non-white canvas or SVG content.	Yes	Yes	Yes
Speed Index (SI)	A timing metric that is calculated by analysing the time that the majority of the user's viewport has rendered.	Yes	No	No
Visually Complete	A timing metric that is calculated by analysing the time that the user's viewport has fully rendered.	Yes	No	No

Runtime#

Runtime metrics are calculated by observing the JavaScript main thread. These metrics will vary due to the CPU conditions of the device.

Metric	Description	Synthetic	CrUX	RUM
Total Blocking Time (TBT)	The total duration of Main thread tasks observed between First Contentful Paint and Time to Interactive.	Yes	No	No
JS Parse & Compile	The duration of the time taken to parse, compile and execute all JavaScript for a page.	Yes	No	No

Lighthouse Scores#

Lighthouse Scores range from 0 to 100. Each score refers to a set of categorised audits and is calculated using a weighted average.

Metric	Description	Synthetic	CrUX	RUM
Performance Score	A number ranging from 0 to 100 evaluating the observed speed characteristics of a given page.	Yes	No	No
Accessibility Score	A number ranging from 0 to 100 evaluating the accessibility properties of a given page.	Yes	No	No
SEO Score	A number ranging from 0 to 100 evaluating the SEO potential of a page from pre-defined audits.	Yes	No	No
Best Practices Score	A number ranging from 0 to 100 evaluating the implementation of a given page in relation to common best practices.	Yes	No	No

Request#

Request metrics relate to the over the wire transfer of HTML, Images, JavaScript, CSS, Font, JSON and Video assets. Each asset type is categorised by mime-type analysis.

These metrics are useful for setting budgets against and can quickly highlight common speed issues like when compression is accidentally turned off, a large library has been added, or when unoptimised images are added via CMS.

Metric	Description	Synthetic	CrUX	RUM
Time to First Byte (TTFB)	The time taken from the request being made until the first byte arriving back to the browser.	Yes	Yes	Yes
Round Trip Time	The time it takes for a network request to travel from the user's browser to the server and back.	Yes	Yes	No
Response time	TTFB plus the time taken for the entire request to be transferred from the server to the client.	Yes	No	No
Total Page Transferred	The total filesize of all page assets transferred during the page load.	Yes	No	No
Total JavaScript Transferred	The total filesize of JavaScript assets transferred during the page load.	Yes	No	No
Number of requests (Asset Count)	The total number of requests.	Yes	No	No
Total HTML Transferred	The total filesize of HTML assets transferred during the page load.	Yes	No	No
Total Image Transferred	The total filesize of Image (JPEG, PNG, GIF, WebP, AVIF and SVG) assets transferred during the page load.	Yes	No	No
Total CSS Transferred	The total filesize of CSS assets transferred during the page load.	Yes	No	No
Total Font Transferred	The total filesize of Font (EOT, TTF, WOFF and WOFF2) assets transferred during the page load.	Yes	No	No
Total JSON Transferred	The total filesize of JSON assets transferred during the page load.	Yes	No	No
Total Video Transferred	The total filesize of Video assets transferred during the page load.	Yes	No	No
Domain Lookup Time	The time taken in DNS lookup for the page.	Yes	No	No
Server Connection Time	The time all samples spent in establishing a TCP connection to the page.	Yes	No	No
SSL Time	The time it takes the browser to handle SSL negotiation for the main/document HTML request	Yes	No	No
HTML Download Time	TTFB plus the time taken to download the HTML.	Yes	No	No

Byte size#

Byte size metrics relate to the uncompressed size of HTML, Images, JavaScript, CSS, Font, JSON, and Video assets once they have been delivered to the client.

Metric	Description	Synthetic	CrUX	RUM
Total HTML Size in Bytes	The total, uncompressed filesize of HTML assets transferred during the page load.	Yes	No	No
Total JavaScript Size in Bytes	The total, uncompressed filesize of JavaScript assets transferred during the page load.	Yes	No	No
Total Image Size in Bytes	The total, uncompressed filesize of Image (JPEG, PNG, GIF, WebP, AVIF and SVG) assets transferred during the page load.	Yes	No	No
Total Font Size in Bytes	The total, uncompressed filesize of Font (EOT, TTF, WOFF and WOFF2) assets transferred during the page load.	Yes	No	No
Total Page Size in Bytes	The total, uncompressed filesize of all page assets transferred during the page load.	Yes	No	No
Total CSS Size in Bytes	The total, uncompressed filesize of CSS assets transferred during the page load.	Yes	No	No
Total JSON Size in Bytes	The total, uncompressed filesize of JSON assets transferred during the page load.	Yes	No	No
Total Video Size in Bytes	The total, uncompressed filesize of Video assets transferred during the page load.	Yes	No	No

Third Party#

Third Party metrics provide an overview of how external code contributes to your Site's speed, which often might have a drastic impact on page speed.

These metrics are useful for setting budgets against and can quickly highlight common speed issues like adding a large external tool or unexpected changes to an already used tool or library.

Metric	Description	Synthetic	CrUX	RUM
Number of Third Party Providers	The total number of identified third party providers existing on a page.	Yes	No	No
Total Third Party Main Thread Execution Time	The total duration of main thread tasks prompted by third party script.	Yes	No	No
Total Third Party Code Transferred	The total filesize of third party JavaScript assets transferred during the page load.	Yes	No	No

Historical (deprecated) metrics#

These metrics were used in the early days of site speed monitoring. They're not relevant because they provided not only unreliable measurements, but also more modern metrics more accurately describe the user experience. Calibre monitors these metrics strictly for historical reference. These metrics are not recommended for tracking.

Metric	Description
onDomContentloaded	The browser emits `onDomContentloaded` when the document has been parsed and the DOM has been constructed.
onLoad	The browser emits `onLoad` when all initial resources have finished loading. This includes HTML, Images, CSS, JavaScript and Video media.
First Paint	The time at which the first page paint event occurred.
First Meaningful Paint	The time at which the largest area of above-the-fold content was painted to the screen.
PWA Score	A number ranging from 0 to 100 evaluating if a page meets the Progressive Web App definition guidelines.
Time to Interactive (TTI)	The moment at which the main thread has had 5 seconds with no network activity or JavaScript long tasks.