Web Performance Glossary

A

AVIF

A modern image format based on the AV1 video codec. ~50% smaller than JPEG at equivalent quality. Browser support is 93%+.

async (script attribute)

Tells the browser to download the script in parallel with HTML parsing and execute as soon as it's ready. Doesn't preserve order between scripts.

B

Brotli

A compression algorithm that produces smaller files than gzip. Supported by all current browsers; should be enabled on every server.

C

Cache-Control

An HTTP response header telling browsers and CDNs how long to cache a resource. See our caching headers guide.

CDN (Content Delivery Network)

A network of servers distributed geographically that cache and serve content from nodes close to the user. Cloudflare, Fastly, Akamai, and Bunny are common CDNs.

CLS (Cumulative Layout Shift)

A Core Web Vital measuring how much content shifts unexpectedly during page load. Good is under 0.1. See our CLS guide.

Code Splitting

Breaking a large JavaScript bundle into smaller chunks that load on demand. Reduces initial bundle size and time-to-interactive.

Core Web Vitals

The three Google performance metrics that influence rankings: LCP (loading), CLS (visual stability), and INP (responsiveness).

Critical CSS

The minimum CSS needed to render above-the-fold content. Inlined in the head to avoid render-blocking on the main stylesheet.

CrUX (Chrome User Experience Report)

Google's public dataset of real-user performance data, used to compute the field-data Core Web Vitals shown in Search Console and PageSpeed Insights.

D

defer (script attribute)

Tells the browser to download the script in parallel with HTML parsing and execute after parsing finishes. Preserves script order. Standard for non-blocking script loading.

DNS Lookup

Translating a hostname to an IP address. Each unique third-party domain costs a DNS lookup; dns-prefetch can warm them up.

E

ETag

An HTTP header containing a unique identifier for a resource version. Used for cache revalidation — server returns 304 Not Modified if the ETag matches.

F

FCP (First Contentful Paint)

Time from page load start to when any content first paints. Less critical than LCP but a useful early indicator.

fetchpriority

An HTML attribute (fetchpriority="high") telling the browser to prioritise downloading a resource. Used on LCP images and critical fonts.

FID (First Input Delay)

The previous Core Web Vital for responsiveness, replaced by INP in March 2024. Measured only the first interaction's delay.

FOIT (Flash of Invisible Text)

The default behaviour of web fonts: text is invisible until the font loads. Avoid with font-display: swap.

FOUT (Flash of Unstyled Text)

The result of font-display: swap: fallback font shows immediately, swaps to custom when ready. Causes a brief layout shift unless metrics match.

G

Gzip

A compression algorithm supported by every browser. Always enable; brotli is better but gzip is the safe baseline.

H

HTTP/2 / HTTP/3

Modern HTTP protocols supporting multiplexing (multiple requests over one connection) and header compression. HTTP/3 uses QUIC for further latency reduction. Both should be enabled on modern servers.

Hydration

The process by which a server-rendered HTML page becomes interactive in the browser when JavaScript runs. Slow hydration is a common cause of high INP.

I

INP (Interaction to Next Paint)

The Core Web Vital measuring how long pages take to respond to user interaction. Good is under 200ms. See our INP guide.

Image CDN

A CDN that serves images with on-the-fly transformations — resizing, format conversion, compression. Cloudflare Images, Imgix, Cloudinary are common.

L

Lazy Loading

Deferring resource loading until needed. <img loading="lazy"> defers off-screen images. Don't use on the LCP element.

LCP (Largest Contentful Paint)

The Core Web Vital measuring when the largest visible element finishes rendering. Good is under 2.5s. See our LCP guide.

Lighthouse

Google's open-source automated audit tool for performance, accessibility, SEO, and best practices. Built into Chrome DevTools.

M

Main Thread

The single thread browsers use for layout, painting, and most JavaScript execution. When the main thread is busy, interactions are blocked.

Minification

Removing whitespace, comments, and shortening identifiers in JS/CSS to reduce file size. Standard build-step.

N

Network Waterfall

A visualisation of resource loading over time, showing what loads when. Available in Chrome DevTools Network panel; essential for performance debugging.

P

PageSpeed Insights

Google's free performance audit tool. Combines Lighthouse lab data with field data from CrUX.

Preload

An HTML hint (<link rel="preload">) telling the browser to fetch a resource early. Used for critical fonts, LCP images, and other render-critical resources.

Preconnect

An HTML hint (<link rel="preconnect">) telling the browser to establish a connection to a domain early, saving DNS lookup, TCP handshake, and TLS handshake time.

R

Render-Blocking

Resources (CSS, synchronous JS) that the browser must process before painting. Eliminate or defer to improve first paint. See our render-blocking guide.

Responsive Images

Images served at different resolutions for different screen sizes, using srcset and sizes. Avoids serving desktop-sized images to mobile devices.

S

Service Worker

A script that runs in the background separate from the page, used for advanced caching strategies, offline support, and background sync.

SSR (Server-Side Rendering)

Rendering HTML on the server before sending to the browser. Improves LCP and SEO compared to client-only rendering.

T

TBT (Total Blocking Time)

Lab metric measuring how long the main thread was blocked between FCP and Time to Interactive. Strongly correlates with INP.

TTFB (Time to First Byte)

Time from request to first byte of response. Affected by server speed, network latency, and CDN coverage. Should be under 600ms.

TTI (Time to Interactive)

Time until the page is reliably responsive to input. Closely related to INP but measured at the page level rather than per-interaction.

Tree Shaking

Removing unused code from a JavaScript bundle at build time. Reduces shipped JS size dramatically when many libraries' features aren't used.

V

Viewport

The visible portion of the page in the browser window. "Above the fold" content is in the viewport on first load.

W

WebP

Google's modern image format. ~25-35% smaller than JPEG at equivalent quality. 97%+ browser support.

Web Worker

A JavaScript thread separate from the main thread, useful for offloading heavy computation without blocking interactions.