Performance
Why sites feel fast or slow: Core Web Vitals, Lighthouse, images, fonts and caching.
60 fps
60 fps means the screen updates sixty times a second, the baseline for motion that looks smooth; each frame has about 16 milliseconds to be ready.
Back/forward cache
The back/forward cache keeps a complete snapshot of a page in memory so pressing Back or Forward restores it instantly instead of reloading.
Compression
Compression shrinks text files like HTML, CSS and JavaScript as the server sends them, using Gzip or Brotli, and the browser expands them on arrival.
Core Web Vitals
Core Web Vitals are Google's three user-experience metrics: Largest Contentful Paint for loading, Interaction to Next Paint for responsiveness and Cumulative Layout Shift for visual stability.
Critical CSS
Critical CSS is the minimal set of styles needed to render the first visible part of a page, inlined in the HTML so the page can draw immediately.
Cumulative Layout Shift
Cumulative Layout Shift measures how much page content moves unexpectedly while loading; good is below 0.1.
First Contentful Paint
First Contentful Paint is the moment the browser first draws any text or image from the page, the first sign to a visitor that something is happening.
Image optimisation
Image optimisation is serving images at the right size, format and quality for each device, so pages load fast without visibly losing quality.
Interaction to Next Paint
Interaction to Next Paint measures how quickly a page visibly responds after a tap, click or key press, across the whole visit; good is 200 milliseconds or less.
Jank
Jank is visible stutter when a page cannot draw frames in time, making scrolling, animation or typing feel choppy.
Largest Contentful Paint
Largest Contentful Paint measures how long it takes for the largest visible element, usually the hero image or headline, to appear; good is 2.5 seconds or less.
Lighthouse
Lighthouse is Google's open-source auditing tool that scores a page from 0 to 100 on performance, accessibility, best practices and SEO, with specific fixes.
Minification
Minification removes whitespace, comments and long names from code so HTML, CSS and JavaScript files are smaller and download faster.
Page speed
Page speed is how quickly a web page loads and becomes usable, measured by metrics like Largest Contentful Paint, Interaction to Next Paint and layout stability.
PageSpeed Insights
PageSpeed Insights is Google's free tool that shows a page's real-user Core Web Vitals, where available, alongside a Lighthouse lab audit.
Performance budget
A performance budget is a set limit, such as total JavaScript size or LCP time, that a site is not allowed to exceed, checked automatically before release.
Preload and prefetch
Preload and prefetch are hints that tell the browser to fetch a resource early: preload for something the current page needs now, prefetch for something the next page will need.
Render-blocking resources
Render-blocking resources are stylesheets and scripts the browser must download and process before it can draw the page, delaying the first paint.
Third-party scripts
Third-party scripts are code loaded from other companies, such as analytics, ads, chat widgets and embeds, and are among the most common causes of slow pages.
Time to First Byte
Time to First Byte is how long the browser waits after requesting a page before the first byte of the response arrives; it reflects server and network speed.
Web font loading
Web font loading is how a page fetches and swaps in custom fonts; done badly it hides text or shifts layout, done well text shows immediately and swaps smoothly.
WebP and AVIF
WebP and AVIF are modern image formats that compress photos and graphics much smaller than JPEG and PNG at similar visual quality, supported by current browsers.