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

Optimized images load faster and consume less cellular data

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

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

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

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

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

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

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

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

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

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

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

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 served over 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

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

Minifying JavaScript files can reduce payload sizes and script parse time

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

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

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

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

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

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

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

This is the largest contentful element painted within the viewport

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

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

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

Minifying CSS files can reduce network payload sizes

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

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

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

Consider using `<link rel=preload>` to prioritize fetching resources that are currently requested later in page load