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

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

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

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

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

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

This is the largest contentful element painted within the viewport

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

Optimized images load faster and consume less cellular data

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

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

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

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

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

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

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

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

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

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

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

Largest Contentful Paint marks the time at which the largest 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)

Minifying CSS files can reduce network payload sizes

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

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

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

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

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

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

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