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

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

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

Remove unused JavaScript to reduce bytes consumed by network activity

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

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

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

All sites should be protected with HTTPS, even ones that don't handle sensitive data. This includes avoiding [mixed content](https://developers.google.com/web/fundamentals/security/prevent-mixed-content/what-is-mixed-content), where some resources are loaded over HTTP despite the initial request being servedover HTTPS. HTTPS prevents intruders from tampering with or passively listening in on the communications between your app and your users, and is a prerequisite for HTTP/2 and many new web platform APIs

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

First CPU Idle marks the first time at which the page's main thread is quiet enough to handle input. [Learn more](https://web.dev/first-cpu-idle/).

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

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

Image formats like JPEG 2000, JPEG XR, and WebP often provide better compression than PNG or JPEG, which means faster downloads and less data consumption

A fast page load over a cellular network ensures a good mobile user experience

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

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

This is the largest contentful element painted within the viewport

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

Estimated Input Latency is an estimate of how long your app takes to respond to user input, in milliseconds, during the busiest 5s window of page load. If your latency is higher than 50 ms, users may perceive your app as laggy

Minifying JavaScript files can reduce payload sizes and script parse time

Large JavaScript libraries can lead to poor performance. Prefer smaller, functionally equivalent libraries to reduce your bundle size

First Contentful Paint marks the time at which the first 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

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)

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

Optimized images load faster and consume less cellular data

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

Redirects introduce additional delays before the page can be loaded

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

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

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

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

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

Remove dead rules from stylesheets and defer the loading of CSS not used for above-the-fold content to reduce unnecessary bytes consumed by network activity

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

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

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

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

Optimized images load faster and consume less cellular data

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

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

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

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

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

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

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

Image formats like JPEG 2000, JPEG XR, and WebP often provide better compression than PNG or JPEG, which means faster downloads and less data consumption

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

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

Redirects introduce additional delays before the page can be loaded

Remove unused JavaScript to reduce bytes consumed by network activity

Estimated Input Latency is an estimate of how long your app takes to respond to user input, in milliseconds, during the busiest 5s window of page load. If your latency is higher than 50 ms, users may perceive your app as laggy

First CPU Idle marks the first time at which the page's main thread is quiet enough to handle input. [Learn more](https://web.dev/first-cpu-idle/).

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

A fast page load over a cellular network ensures a good mobile user experience

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

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

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

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

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

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

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

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

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

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

All sites should be protected with HTTPS, even ones that don't handle sensitive data. This includes avoiding [mixed content](https://developers.google.com/web/fundamentals/security/prevent-mixed-content/what-is-mixed-content), where some resources are loaded over HTTP despite the initial request being servedover HTTPS. HTTPS prevents intruders from tampering with or passively listening in on the communications between your app and your users, and is a prerequisite for HTTP/2 and many new web platform APIs

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

Remove dead rules from stylesheets and defer the loading of CSS not used for above-the-fold content to reduce unnecessary bytes consumed by network activity

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

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

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

This is the largest contentful element painted within the viewport

Large JavaScript libraries can lead to poor performance. Prefer smaller, functionally equivalent libraries to reduce your bundle size

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

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

Minifying JavaScript files can reduce payload sizes and script parse time

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

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

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)