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    • Overview

    This guide includes all of the metrics that Calibre observes and calculates. You’ll learn how metrics are captured, what they indicate and the factors that influence them.

    Tip
    We display Core Web Vitals across Calibre as the default metric set (Largest Contentful Paint, Cumulative Layout Shift and Total Blocking Time). You can configure metrics for Pull Request Reviews, Pulse screen and Pages Leaderboard.

    Paint

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

    MetricDescription
    Largest Contentful Paint (LCP)The time at which the largest image, video or text element was painted to the screen.
    Cumulative Layout Shift (CLS)The sum of scores from all of the unstable elements that move around during page load.
    First Contentful Paint (FCP)The time at which the browser rendered any text, image, non-white canvas or SVG content.
    Speed Index (SI)A timing metric that is calculated by analysing the time that the majority of the user’s viewport has rendered.
    Visually CompleteA timing metric that is calculated by analysing the time that the user’s viewport has fully rendered.

    Runtime

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

    MetricDescription
    Time to Interactive (TTI)The moment at which the main thread has had 5 seconds with no network activity or JavaScript long tasks.
    Total Blocking Time (TBT)The total duration of Main thread tasks observed between First Contentful Paint and Time to Interactive.
    JS Parse & CompileThe duration of the time taken to parse, compile and execute all JavaScript for a page.
    Max Potential First Input DelayThe duration of the worst-case scenario time taken to respond to a user interaction.

    Lighthouse Scores

    Lighthouse scores range from 0 to 100. Each metric refers to categorised audits and are calculated using a weighted average.

    MetricDescription
    Performance ScoreA number ranging from 0 to 100 evaluating the observed speed characteristics of a given page.
    Accessibility ScoreA number ranging from 0 to 100 evaluating the accessibility properties of a given page.
    SEO ScoreA number ranging from 0 to 100 evaluating the SEO potential of a page from pre-defined audits.
    Best Practices ScoreA number ranging from 0 to 100 evaluating the implementation of a given page in relation to common best practices.
    PWA ScoreA number ranging from 0 to 100 evaluating if a page meets the Progressive Web App definition guidelines.

    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.

    MetricDescription
    Time to First Byte (TTFB)The time taken from the request being made until the first byte arriving back to the browser.
    Response timeTTFB plus the time taken for the entire request to be transferred from the server to the client.
    Total Page TransferredThe total filesize of all page assets transferred during the page load.
    Total JavaScript TransferredThe total filesize of JavaScript assets transferred during the page load.
    Number of requests (Asset Count)The total number of requests.
    Total HTML TransferredThe total filesize of HTML assets transferred during the page load.
    Total Image TransferredThe total filesize of Image (JPEG, PNG, GIF, WebP, AVIF and SVG) assets transferred during the page load.
    Total CSS TransferredThe total filesize of CSS assets transferred during the page load.
    Total Font TransferredThe total filesize of Font (EOT, TTF, WOFF and WOFF2) assets transferred during the page load.
    Total JSON TransferredThe total filesize of JSON assets transferred during the page load.
    Total Video TransferredThe total filesize of Video assets transferred during the page load.
    Domain Lookup TimeThe time taken in DNS lookup for the page.
    Server Connection TimeThe time all samples spent in establishing a TCP connection to the page.
    SSL TimeThe time it takes the browser to handle SSL negotiation for the main/document HTML request.
    HTML Download TimeTTFB plus the time taken to download the HTML.

    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.

    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.

    MetricDescription
    Total HTML Size in BytesThe total, uncompressed filesize of HTML assets transferred during the page load.
    Total JavaScript Size in BytesThe total, uncompressed filesize of JavaScript assets transferred during the page load.
    Total Image Size in BytesThe total, uncompressed filesize of Image (JPEG, PNG, GIF, WebP, AVIF and SVG) assets transferred during the page load.
    Total Font Size in BytesThe total, uncompressed filesize of Font (EOT, TTF, WOFF and WOFF2) assets transferred during the page load.
    Total Page Size in BytesThe total, uncompressed filesize of all page assets transferred during the page load.
    Total CSS Size in BytesThe total, uncompressed filesize of CSS assets transferred during the page load.
    Total JSON Size in BytesThe total, uncompressed filesize of JSON assets transferred during the page load.
    Total Video Size in BytesThe total, uncompressed filesize of Video assets transferred during the page load.

    Third Party

    Third Party metrics provide an overview of how external code contributes to your Sites speed, which in many cases 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 already used tool bundle.

    MetricDescription
    Number of Third Party ProvidersThe total number of identified third party providers existing on a page.
    Total Third Party Main Thread Execution TimeThe total duration of main thread tasks prompted by third party script.
    Total Third Party Code TransferredThe total filesize of third party JavaScript assets transferred during the page load.

    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.

    MetricDescription
    onDomContentloadedThe browser emits onDomContentloaded when the document has been parsed and the DOM has been constructed.
    onLoadThe browser emits onLoad when all initial resources have finished loading. This includes HTML, Images, CSS, JavaScript and Video media.
    First PaintThe time at which the first page paint event occurred.
    First Meaningful PaintThe time at which the largest area of above-the-fold content was painted to the screen.
    Visually Complete 85%A timing metric that is calculated by analysing the time that the user’s viewport has been 85% rendered.
    First CPU IdleThe moment at which the main thread has no active long tasks since First Contentful Paint.