Performance terms, explained simply
A plain-English glossary of website speed and performance jargon — from bandwidth to Time to First Byte to render-blocking resources.
Performance discussions come with a lot of jargon. This glossary defines the most common terms in plain English.
Quick summary
Use this page as a reference when you encounter an unfamiliar term in a PageSpeed report, a developer conversation, or a hosting dashboard. Terms are listed alphabetically.
A
Above the fold The part of a web page that is visible without scrolling. Images and content above the fold are loaded with high priority because visitors see them immediately.
Asset Any file a web page needs to load: images, CSS files, JavaScript files, fonts, videos. Reducing asset size and count is central to performance optimization.
B
Bandwidth The amount of data that can be transferred over a connection in a given time. Higher bandwidth means more data can flow faster. Your hosting plan usually has a bandwidth limit — the total data served to visitors per month.
Browser caching When a browser saves copies of files from websites you have visited. On your next visit, the browser loads those files from your own computer instead of re-downloading them, making the site feel faster.
C
Cache / Caching Storing a saved copy of content so it can be served quickly without being rebuilt from scratch. See Caching explained.
Cache purge / Cache clear Deleting cached copies so the next visitor gets a fresh version. Necessary after making site updates.
CDN (Content Delivery Network) A network of servers in many locations that stores copies of your site's files. Visitors get files from the nearest server, reducing travel time. See How a CDN speeds up your site.
CLS (Cumulative Layout Shift) One of Google's Core Web Vitals. Measures how much the page layout moves unexpectedly while loading. A low score (under 0.1) is good.
Compression Reducing the file size of an image or other asset. Lossless compression reduces size without any quality loss. Lossy compression reduces size more aggressively but may reduce visible quality slightly.
Core Web Vitals Google's three key performance measurements: LCP (loading speed), INP (responsiveness), and CLS (visual stability). Used as a search ranking signal. See Core Web Vitals explained.
Critical path The sequence of resources that must load before a page can be displayed. Shortening the critical path is a key performance goal.
D
DOM (Document Object Model) The browser's internal representation of the page structure. A complex page with thousands of elements has a large DOM, which can slow rendering.
F
FCP (First Contentful Paint) When the first text or image appears on screen. A good FCP is under 1.8 seconds.
Field data Performance measurements collected from real visitors using real devices in real conditions. Contrast with lab data. Field data is more representative of your actual visitors' experience.
FOIT (Flash of Invisible Text) When text is hidden while a web font downloads, then appears suddenly. A poor user experience that can be avoided with font-display settings.
FOUT (Flash of Unstyled Text) When text appears in a fallback font first, then switches to the web font when it arrives. Less disruptive than FOIT, and generally preferred.
H
HTTP/2 A modern version of the web's communication protocol. HTTP/2 allows multiple files to be requested simultaneously over one connection, which is faster than the older HTTP/1.1 that required separate connections for each file.
I
INP (Interaction to Next Paint) One of Google's Core Web Vitals. Measures how quickly the page responds to clicks, taps, and keyboard input. Under 200 ms is good.
J
JavaScript A programming language that makes web pages interactive. Heavy JavaScript is the most common cause of slow INP scores and poor mobile performance.
JPEG / JPG A common image format good for photographs. Lossy compression — some detail is removed to reduce file size. Being replaced by WebP for most uses.
L
Lab data Performance measurements from a simulated test in a controlled environment. Consistent and useful for identifying problems, but does not reflect the variability of real visitors.
Latency The delay between sending a request and receiving the first byte of a response. Measured in milliseconds. High latency makes pages feel slow even on fast connections.
Lazy loading Deferring the loading of images and other assets until they are close to the viewport (the visible area). Images below the fold do not load until the visitor scrolls toward them.
LCP (Largest Contentful Paint) One of Google's Core Web Vitals. Measures when the largest visible piece of content loads. Under 2.5 seconds is good.
Lighthouse Google's open-source tool for measuring performance, accessibility, SEO, and best practices. It powers the "lab data" section of PageSpeed Insights.
M
Minification Removing unnecessary characters (spaces, comments, line breaks) from code files (CSS, JavaScript) to reduce their file size. Has no effect on how the code runs.
P
PageSpeed Insights Google's free speed testing tool. Shows both field data (from real visitors) and lab data (from a simulated test), along with specific improvement recommendations. Available at pagespeed.web.dev.
PNG An image format that supports transparency. Produces larger files than JPEG or WebP for photographs; good for logos and illustrations with transparency.
Preload An instruction to the browser to download a resource (like a font or hero image) early, before it would normally be discovered in the page. Reduces delay for critical assets.
R
Render-blocking A file (CSS or JavaScript) that the browser must fully download and process before it can display the page. Render-blocking resources delay First Contentful Paint.
Responsive design A website design that adapts to different screen sizes. A responsive site serves appropriately-sized layouts and images to phones, tablets, and desktops.
S
Squoosh A free, browser-based image compression tool by Google. Useful for compressing and converting images before uploading. Available at squoosh.app.
SVG (Scalable Vector Graphic) An image format based on code rather than pixels. Scales to any size with no quality loss. Ideal for logos and icons.
T
TTFB (Time to First Byte) The time between a browser requesting a page and receiving the first byte of data from the server. A direct measure of server responsiveness. Under 800 ms is good.
Tree shaking A developer technique that removes unused code from JavaScript bundles, reducing file size. Performed at build time, not something you configure manually.
W
WebP A modern image format developed by Google. Typically 25–35% smaller than equivalent JPEG files at similar quality. Widely supported by all modern browsers.
Web font A typeface downloaded from a server rather than built into the visitor's device. Web fonts add personality to a site but require extra file downloads.
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