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

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

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

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 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

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

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

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

Deprecated APIs will eventually be removed from the browser

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

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

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

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

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

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

Minifying JavaScript files can reduce payload sizes and script parse time

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

This is the largest contentful element painted within the viewport

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

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

Minifying CSS files can reduce network payload sizes

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

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

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

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

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

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

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

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

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

Remove unused JavaScript to reduce bytes consumed by network activity

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)

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

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

Large GIFs are inefficient for delivering animated content. Consider using MPEG4/WebM videos for animations and PNG/WebP for static images instead of GIF to save network bytes

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