Google Page Speed Insights (PSI)

PageSpeed Insights (PSI) reports on the performance of a page on both mobile and desktop devices, and provides suggestions on how that page may be improved. PSI provides both lab and field data about a page.

Lab data is useful for debugging performance issues, as it is collected in a controlled environment. However, it may not capture real-world bottlenecks. Field data is useful for capturing true, real-world user experience - but has a more limited set of metrics.

Performance Score

A score of 90 or above is considered good. 50 to 90 is a score that needs improvement, and below 50 is considered poor.

Real-World Field Data

When PSI is given a URL, it will look it up in the Chrome User Experience Report (CrUX) dataset. If available, PSI reports the following metric data for the origin and potentially the specific page URL:

  1. First Contentful Paint (FCP)
  2. First Input Delay (FID)
  3. Largest Contentful Paint (LCP)
  4. Cumulative Layout Shift (CLS)
Metric Good Needs Improvement Poor
FCP [0, 1000ms] (1000ms, 3000ms] over 3000ms
FID [0, 100ms] (100ms, 300ms] over 300ms
LCP [0, 2500ms] (2500ms, 4000ms] over 4000ms
CLS [0, 0.1] (0.1, 0.25] over 0.25

Core Web Vitals: Core Web Vitals are a common set of signals critical to all web experiences. The Core Web Vitals metrics are FID, LCP, and CLS, with their respective thresholds. A page passes the Core Web Vitals assessment if the 75th percentiles of all three metrics are Good. Otherwise, the page does not pass the assessment.

First Contentful Paint (FCP)

The First Contentful Paint (FCP) metric measures the time from when the page starts loading to when any part of the page's content is rendered on the screen. It is an important, user-centric metric for measuring perceived load speed because it marks the first point in the page load timeline where the user can see anything on the screen - a fast FCP helps reassure the user that something is happening.

Largest Contentful Paint (LCP)

The Largest Contentful Paint (LCP) metric reports the render time of the largest image or text block visible within the viewport. It is an important, user-centric metric for measuring perceived load speed because it marks the point in the page load timeline when the page's main content has likely loaded - a fast LCP helps reassure the user that the page is useful.

The size of the element reported for Largest Contentful Paint is typically the size that's visible to the user within the viewport.

First Input Delay (FID)

FID measures the time from when a user first interacts with a page (i.e. when they click a link, tap on a button, or use a custom, JavaScript-powered control) to the time when the browser is actually able to begin processing event handlers in response to that interaction. It is an important, user-centric metric for measuring load responsiveness because it quantifies the experience users feel when trying to interact with unresponsive pages - a low FID helps ensure that the page is usable.

Cumulative Layout Shift (CLS)

CLS measures the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of the page. It is an important, user-centric metric for measuring visual stability because it helps quantify how often users experience unexpected layout shifts - a low CLS helps ensure that the page is delightful.

Lab data

PSI uses Lighthouse to analyze the given URL, generating a performance score that estimates the page's performance on different metrics, including:

  1. First Contentful Paint
  2. Largest Contentful Paint
  3. Speed Index
  4. Cumulative Layout Shift
  5. Time to Interactive
  6. Total Blocking Time

Audits

Lighthouse separates its audits into three sections:

  1. Opportunities provide suggestions how to improve the page's performance metrics. Each suggestion in this section estimates how much faster the page will load if the improvement is implemented.

  2. Diagnostics provide additional information about how a page adheres to best practices for web development.

  3. Passed Audits indicates the audits that have been passed by the page.

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