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

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

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

These DOM elements contribute most to the CLS of the page

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

This is the largest contentful element painted within the viewport

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

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

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

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

If your robots.txt file is malformed, crawlers may not be able to understand how you want your website to be crawled or indexed

Remove unused JavaScript to reduce bytes consumed by network activity

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/).

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

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

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

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

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

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

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

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

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

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

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

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

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

Text-based resources should be served with compression (gzip, deflate or brotli) to minimize total network bytes

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

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

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)

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

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

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

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

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

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

This is the largest contentful element painted within the viewport

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