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.

Above the fold

The part of a web page that is visible without scrolling. This is the content you want to load first, because visitors see it immediately — and it is usually where the Largest Contentful Paint element sits.

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.

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.

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. Google's threshold for good is 0.1 or less; over 0.25 is poor.

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). Google says Core Web Vitals are used by its ranking systems, while also noting there is no single "page experience" ranking signal — they are one input among many. 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.

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.

FCP (First Contentful Paint)

When the first text or image appears on screen. Google's threshold for good is 1.8 seconds or less; over 3 seconds is poor.

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. Both FOIT and FOUT are trade-offs, and web.dev declines to name a single winner: the swap behind FOUT delays text the least, which makes it the usual choice when getting words on screen matters most, but the switch itself can shift the layout.

HTTP/2

A modern version of the web's communication protocol. HTTP/2 uses multiplexing, so several requests and responses can be in flight at the same time over a single connection. HTTP/1.1 can reuse one connection for several files (a "persistent" or keep-alive connection), but it works through them in order, so browsers open several connections at once to compensate.

INP (Interaction to Next Paint)

One of Google's Core Web Vitals. Measures how quickly the page responds to clicks, taps, and keyboard input. Google's threshold for good is 200 ms or less; over 500 ms is poor.

JavaScript

A programming language that makes web pages interactive. Google describes JavaScript as often the primary driver of how interactive a page feels, so heavy JavaScript is a 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. WebP usually produces a smaller file for the same photograph.

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 travel time on a connection: how long a request takes to reach the server and the answer to start coming back. Measured in milliseconds. High latency makes pages feel slow even when the connection has plenty of bandwidth, which is why a CDN that shortens the distance helps.

Lazy loading

Deferring the loading of images and other assets until they are close to the viewport (the visible area). It is switched on with a loading="lazy" instruction on the image, which tells the browser not to download it until it is in or near 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. Google's threshold for good is 2.5 seconds or less; over 4 seconds is poor.

Lighthouse

Google's open-source tool for measuring performance, accessibility, SEO, and best practices. It powers the "lab data" section of PageSpeed Insights.

Minification

Removing unnecessary characters (spaces, comments, line breaks) from code files (CSS, JavaScript) to reduce their file size. Done correctly it does not change how the code runs, but an aggressive minifier — especially one that also merges files together — can break a layout or a script, which is why it is worth testing afterwards.

PageSpeed Insights

Google's free speed testing tool. Shows both field data (from real visitors over the previous 28 days, via the Chrome User Experience Report) and lab data (from a single simulated Lighthouse 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.

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.

Squoosh

A free, browser-based image compression tool from Google's Chrome team. Useful for compressing and converting images before uploading, and your images never leave your own machine — it does the work locally. 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.

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. Google's guide is that most sites should aim for 0.8 seconds or less, with anything over 1.8 seconds counted as poor. TTFB is not itself a Core Web Vital, so hitting the target matters less than whether it is holding back LCP.

Tree shaking

A developer technique that removes unused code from JavaScript bundles, reducing file size. Performed at build time, not something you configure manually.

WebP

A modern image format developed by Google. Google's own figures: a lossy WebP is 25–34% smaller than a comparable JPEG at equivalent quality, and a lossless WebP is 26% smaller than a PNG. Supported natively by Chrome, Safari, Firefox, Edge and Opera.

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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