Polyfills and transforms enable legacy browsers to use new JavaScript features. However, many aren't necessary for modern browsers. For your bundled JavaScript, adopt a modern script deployment strategy using module/nomodule feature detection to reduce the amount of code shipped to modern browsers, while retaining support for legacy browsers

Reduce unused JavaScript and defer loading scripts until they are required to decrease bytes consumed by network activity

Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds

Large network payloads cost users real money and are highly correlated with long load times

Consider lazy-loading offscreen and hidden images after all critical resources have finished loading to lower time to interactive

Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this

The maximum potential First Input Delay that your users could experience is the duration of the longest task

This is the largest contentful element painted within the viewport

Serve images that are appropriately-sized to save cellular data and improve load time

The Critical Request Chains below show you what resources are loaded with a high priority. Consider reducing the length of chains, reducing the download size of resources, or deferring the download of unnecessary resources to improve page load

Server latencies can impact web performance. If the server latency of an origin is high, it's an indication the server is overloaded or has poor backend performance

Third-party code can significantly impact load performance. Limit the number of redundant third-party providers and try to load third-party code after your page has primarily finished loading

Image formats like WebP and AVIF often provide better compression than PNG or JPEG, which means faster downloads and less data consumption

Network round trip times (RTT) have a large impact on performance. If the RTT to an origin is high, it's an indication that servers closer to the user could improve performance

Optimized images load faster and consume less cellular data

First Contentful Paint marks the time at which the first text or image is painted

Some third-party scripts may contain known security vulnerabilities that are easily identified and exploited by attackers

Keep the server response time for the main document short because all other requests depend on it

Largest Contentful Paint marks the time at which the largest text or image is painted

Consider instrumenting your app with the User Timing API to measure your app's real-world performance during key user experiences

First Meaningful Paint measures when the primary content of a page is visible

These DOM elements contribute most to the CLS of the page.

Some third-party embeds can be lazy loaded. Consider replacing them with a facade until they are required

To set budgets for the quantity and size of page resources, add a budget.json file

Service worker is the technology that enables your app to use many Progressive Web App features, such as offline, add to homescreen, and push notifications. With proper service worker and manifest implementations, browsers can proactively prompt users to add your app to their homescreen, which can lead to higher engagement

Time to interactive is the amount of time it takes for the page to become fully interactive

Speed Index shows how quickly the contents of a page are visibly populated

Reduce unused rules from stylesheets and defer CSS not used for above-the-fold content to decrease bytes consumed by network activity

Cumulative Layout Shift measures the movement of visible elements within the viewport

Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this

Animations which are not composited can be janky and increase CLS

A large DOM will increase memory usage, cause longer [style calculations](https://developers.google.com/web/fundamentals/performance/rendering/reduce-the-scope-and-complexity-of-style-calculations), and produce costly [layout reflows](https://developers.google.com/speed/articles/reflow)

Lists the longest tasks on the main thread, useful for identifying worst contributors to input delay

Resources are blocking the first paint of your page. Consider delivering critical JS/CSS inline and deferring all non-critical JS/styles

A long cache lifetime can speed up repeat visits to your page

Largest Contentful Paint marks the time at which the largest text or image is painted

Some third-party scripts may contain known security vulnerabilities that are easily identified and exploited by attackers

To set budgets for the quantity and size of page resources, add a budget.json file

Server latencies can impact web performance. If the server latency of an origin is high, it's an indication the server is overloaded or has poor backend performance

This is the largest contentful element painted within the viewport

First Meaningful Paint measures when the primary content of a page is visible

Speed Index shows how quickly the contents of a page are visibly populated

Cumulative Layout Shift measures the movement of visible elements within the viewport

Third-party code can significantly impact load performance. Limit the number of redundant third-party providers and try to load third-party code after your page has primarily finished loading

Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds

Resources are blocking the first paint of your page. Consider delivering critical JS/CSS inline and deferring all non-critical JS/styles

Some third-party embeds can be lazy loaded. Consider replacing them with a facade until they are required

Polyfills and transforms enable legacy browsers to use new JavaScript features. However, many aren't necessary for modern browsers. For your bundled JavaScript, adopt a modern script deployment strategy using module/nomodule feature detection to reduce the amount of code shipped to modern browsers, while retaining support for legacy browsers

Lists the longest tasks on the main thread, useful for identifying worst contributors to input delay

The Critical Request Chains below show you what resources are loaded with a high priority. Consider reducing the length of chains, reducing the download size of resources, or deferring the download of unnecessary resources to improve page load

Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this

Optimized images load faster and consume less cellular data

Reduce unused rules from stylesheets and defer CSS not used for above-the-fold content to decrease bytes consumed by network activity

Keep the server response time for the main document short because all other requests depend on it

Consider instrumenting your app with the User Timing API to measure your app's real-world performance during key user experiences

Time to interactive is the amount of time it takes for the page to become fully interactive

Network round trip times (RTT) have a large impact on performance. If the RTT to an origin is high, it's an indication that servers closer to the user could improve performance

Large network payloads cost users real money and are highly correlated with long load times

Serve images that are appropriately-sized to save cellular data and improve load time

First Contentful Paint marks the time at which the first text or image is painted

A large DOM will increase memory usage, cause longer [style calculations](https://developers.google.com/web/fundamentals/performance/rendering/reduce-the-scope-and-complexity-of-style-calculations), and produce costly [layout reflows](https://developers.google.com/speed/articles/reflow)

First Contentful Paint 3G marks the time at which the first text or image is painted while on a 3G network

Interactive elements like buttons and links should be large enough (48x48px), and have enough space around them, to be easy enough to tap without overlapping onto other elements

Font sizes less than 12px are too small to be legible and require mobile visitors to “pinch to zoom” in order to read. Strive to have >60% of page text ≥12px

The maximum potential First Input Delay that your users could experience is the duration of the longest task

Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this

Service worker is the technology that enables your app to use many Progressive Web App features, such as offline, add to homescreen, and push notifications. With proper service worker and manifest implementations, browsers can proactively prompt users to add your app to their homescreen, which can lead to higher engagement

A long cache lifetime can speed up repeat visits to your page

Image formats like WebP and AVIF often provide better compression than PNG or JPEG, which means faster downloads and less data consumption

Reduce unused JavaScript and defer loading scripts until they are required to decrease bytes consumed by network activity

Animations which are not composited can be janky and increase CLS

Consider lazy-loading offscreen and hidden images after all critical resources have finished loading to lower time to interactive

These DOM elements contribute most to the CLS of the page.