Website speed glossary in plain words

A

• Above the fold. The part of a page that shows on screen before anyone scrolls. It depends on the screen, so a phone and a laptop see different things. The main image usually sits here. See lazy loading.
• Alt text. The words in an image's alt attribute. Screen readers read them aloud, search engines use them, and browsers show them if the picture fails. See image SEO and speed.
• Aspect ratio. The relationship between width and height, such as 16 to 9. Setting width and height lets the browser keep the right shape before the file arrives. See image width and height.
• Async. A script attribute. The file downloads while the page is read, and runs as soon as it arrives, in whatever order the files finish. See defer, async and modules.
• AVIF. A modern image format. For photos, AVIF or WebP with a JPEG fallback is the usual choice. See image formats.

B

• Back/forward cache (bfcache). A browser feature that keeps a whole page in memory when the visitor leaves, so the Back and Forward buttons bring it back at once. See the back/forward cache.
• Bandwidth. How much data a connection can carry in a given time, usually in megabits per second. It is not the same as latency.
• Brotli. A compression method for text files. It usually makes them smaller than gzip does. See text compression.
• Browser cache. The files a visitor's own browser keeps, so that it does not download them again. See caching explained.
• Bundle. One file made by joining many pieces of code. A bundle that holds code for every page makes each page pay for all of it. See bundle size and code splitting.

C

• Cache. A saved copy, kept so that the work of making or fetching it is not done again. A web cache stores responses for later requests. See caching explained.
• Cache-Control. The HTTP header that says how long a copy stays fresh and who may keep it. See the Cache-Control cheat sheet.
• Canonical link. A link in a page's head that names the main address for content that can be reached at several. See canonical tags.
• CDN. A content delivery network: servers in many places that hold copies of your files and serve each visitor from a nearby one. See what is a CDN.
• Chrome UX Report (CrUX). Google's public dataset of how real Chrome users experience sites. It supplies the real-visitor numbers in PageSpeed Insights and Search Console. See the Chrome UX Report.
• CLS. Cumulative Layout Shift. A score for how much visible content moves without warning. 0.1 or less is good. See CLS.
• Compression. Packing a file smaller before it is sent and unpacking it on arrival. Text compression is a server setting. See text compression.
• Core Web Vitals. Three measurements of how a page feels to a real visitor: LCP for loading, INP for responsiveness and CLS for stability. See Core Web Vitals.
• Crawler. A program that fetches pages so a search engine or another service can read them. See robots.txt and noindex.
• Critical CSS. The few style rules needed to draw the first screen, placed in the page itself so the first paint does not wait for the full style sheet. See critical CSS.
• CrUX. See Chrome UX Report.

D

• Defer. A script attribute. The file downloads while the page is read and runs afterwards, in order. See defer, async and modules.
• DNS. The Domain Name System. It turns a domain name into the address of a server. See DNS performance.
• DOM. The Document Object Model: the browser's live model of a page, a tree with a node for each element. See DOM size.
• DOMContentLoaded. An event that fires when the HTML has been read and deferred scripts have run. It does not wait for images.

E

• Edge. A location near the visitor, where a CDN keeps copies of your files.
• ETag. A label the server gives to one version of a file. A cache can ask whether it still holds that version and get a short "yes" back with no file. See HTTP caching headers.
• Event handler. The code that runs when someone clicks, taps or presses a key. How long it takes is part of INP. See how to fix INP.

F

• Facade. A light stand-in, such as a picture with a play button, shown until the visitor asks for the heavy thing. The real player or widget then loads. See third-party scripts.
• FCP. First Contentful Paint: how long a visitor looks at a blank screen before anything appears. 1.8 seconds or less is good. See FCP.
• Fetch priority. The fetchpriority attribute, a hint that one resource matters more or less than others of its kind. See the image attributes cheat sheet.
• Field data. Measurements from real visits to a page, over a period of time. See lab data and field data.
• FID. First Input Delay, the older responsiveness measure. INP replaced it as a Core Web Vital on March 12, 2024. See INP.
• font-display. A CSS setting that says what the browser shows while a web font loads. See web fonts and speed.

G

• GIF. An image format from 1987 with at most 256 colors per frame. Video is usually much smaller for animation. See animated GIFs vs video.
• gzip. A widely supported compression format for text files. See text compression.

H

• Hero image. The large picture at the top of a page. It is often the Largest Contentful Paint element. See LCP.
• hreflang. Link attributes that tell search engines which pages are versions of each other in different languages. See hreflang for English and Spanish.
• HSTS. HTTP Strict Transport Security: a header that tells browsers to always use HTTPS for your site. See HTTPS and security headers.
• HTTP. The language browsers and servers use to ask for and send files.
• HTTP/2 and HTTP/3. Newer versions of HTTP. HTTP/2 sends many requests at once over one connection. HTTP/3 runs over QUIC. See HTTP/2 and HTTP/3.
• HTTPS. HTTP over an encrypted connection. See TLS and HTTPS speed.
• Hydration. The step where a framework attaches its components to HTML the server already made. Until it ends, the page may ignore taps. See hydration.

I

• Indexing. A search engine storing a page so that it can show it in results. See robots.txt and noindex.
• INP. Interaction to Next Paint. It measures how quickly a page visibly responds to a tap, click or key press. 200 milliseconds or less is good. See INP.
• Intrinsic size. The size an image has in pixels before any style changes it. The width and height attributes should match it. See image width and height.

J

• Jank. Sluggishness in a page, usually from long tasks that block the main thread. See break up long tasks.
• jQuery. An older JavaScript library that many sites still load. See remove jQuery.
• JPEG. A lossy image format widely used for photos.
• JSON-LD. The usual way to write structured data: a block of JSON in a script tag. See structured data basics.

L

• Lab data. Measurements from a test that loads a page once, under set conditions. See lab data and field data.
• Latency. The time it takes for a request or an answer to travel across a network. High latency means long waits. See DNS, TLS and connections.
• LCP. Largest Contentful Paint: the time until the biggest image or block of text in view has been drawn. 2.5 seconds or less is good. See LCP.
• Lazy loading. Not downloading something until the visitor is about to see it. See lazy loading.
• Lighthouse. Google's free, open-source tool that tests a page in a controlled copy of Chrome. See Lighthouse.
• Long task. A piece of work that keeps the main thread busy for 50 milliseconds or more. See break up long tasks.
• Lossless and lossy. Lossless compression lets the original be rebuilt exactly. Lossy compression throws some detail away to make the file smaller.

M

• Main thread. The part of the browser that runs your JavaScript and also lays out and draws the page, one thing at a time. See reduce main-thread work.
• Meta description. A short summary of a page, written in its head. Search engines may show it in results. See titles, descriptions and headings.
• Minification. Removing needless characters from code, such as spaces and comments, without changing what it does.
• Mixed content. A page loaded over HTTPS that pulls in some files over plain HTTP. See HTTPS and security headers.
• Mobile-first indexing. Search engines reading the phone version of your pages. See mobile-first indexing.

N

• Network throttling. Slowing a connection on purpose in a test, to copy what visitors on weak connections see. See Chrome DevTools Network panel.
• noindex. A rule that asks search engines to keep a page out of their results. See robots.txt and noindex.

O

• Object cache. A store in memory for the results of database queries, so they need not be asked for again. See WordPress caching.
• Opcode cache. A cache that keeps compiled PHP code in memory, so the server does not read and compile the files on every request. See WordPress caching.
• Origin. The protocol, host name and port of an address, such as https://example.com. Two addresses share an origin only when all three match.

P

• Page cache. A saved, finished copy of a page, served without building it again. See caching explained.
• Page weight. The total number of bytes a page downloads. See page weight and requests.
• PageSpeed Insights. Google's free tool that combines a Lighthouse test with real-visitor data. See PageSpeed Insights explained.
• Percentile. A way to describe the spread of results. At the 75th percentile, three in four visits are at least as good as the mark. See Core Web Vitals.
• Performance budget. A limit you set for page weight, scripts or a metric, so that slow changes are caught before they ship. See automated testing and budgets.
• Preconnect. A hint that opens a connection to another origin early. See resource hints.
• Prefetch. Fetching files for a page the visitor will probably open next, in the background. See speculation rules.
• Preload. A hint that fetches a file the page needs now but would find late. See resource hints.
• Prerender. Loading and running a likely next page in a hidden tab, so the click feels instant. See speculation rules.

Q

• QUIC. The transport protocol that HTTP/3 uses in place of TCP. It was designed for quicker connection setup. See HTTP/2 and HTTP/3.

R

• Raw HTML and rendered HTML. Raw HTML is the text the server sends. The rendered page is the result once the browser has also run the page's scripts. See JavaScript and SEO.
• Real user monitoring (RUM). A small script that measures the speed each visitor gets and sends the numbers to you. See real user monitoring.
• Redirect. An answer that sends the browser to another address. See redirects and status codes.
• Render-blocking. A file that stops the browser from drawing the page until it has been fetched and read. See critical CSS and render-blocking.
• Responsive images. Offering several sizes of one picture so the browser picks the one that fits. See responsive images.
• robots.txt. A file at the root of a site that asks crawlers which paths they may fetch. See robots.txt and noindex.
• Round trip. A message to a server and the answer back. Each new connection costs several. See DNS, TLS and connections.

S

• Schema.org. The shared vocabulary used to describe things in structured data. See structured data basics.
• Server response time. How long a server takes to start answering once a request has reached it. See server response time.
• Service worker. A script between a page and the network that can answer requests from its own cache. See service workers and speed.
• Speed Index. One number for how quickly the visible part of a page fills in. See Speed Index.
• srcset and sizes. Image attributes that list the available files and how wide the picture will be shown. See responsive images.
• SSL. The old name for the security technology now called TLS.
• stale-while-revalidate. A cache setting that lets a slightly old copy be used while a fresh one loads in the background. See the Cache-Control cheat sheet.
• Status code. The three-digit number that starts every server response: 2xx success, 3xx redirect, 4xx request problem, 5xx server failure. See redirects and status codes.
• Structured data. Code that tells search engines what a page is about in a form a machine can read. See structured data basics.
• SVG. An image format that describes a picture as drawing commands in text, so it stays sharp at any size. See SVG for speed.
• Synthetic monitoring. Automated tests that load pages on a schedule under consistent conditions. See automated testing and budgets.

T

• TBT. Total Blocking Time: how long the main thread was too busy to respond while the page loaded. A lab metric. See Total Blocking Time.
• Third-party script. Code from another company that runs on your page, such as analytics, ads, chat and maps. See third-party scripts.
• Time to First Byte (TTFB). The wait from the moment a visitor asks for a page until the first byte of the answer arrives. 0.8 seconds or less is good. See TTFB.
• TLS. Transport Layer Security: the technology that encrypts HTTPS traffic. See TLS and HTTPS speed.
• Tree shaking. Removing code that is never used when a bundle is built. See reduce unused code.

U

• Unused code. JavaScript and CSS that a page downloads but never uses. See reduce unused code.

V

• Vary. A header that tells caches an answer depends on a request header, such as Accept-Encoding. See HTTP caching headers.
• Viewport. The part of a page you can see in the browser window. The viewport tag tells a phone to use its real width. See mobile-first indexing.

W

• Waterfall. A chart that lists each file a page loads as a bar on a timeline. See Chrome DevTools Network panel.
• Web font. A font file that downloads with the page. See web fonts and speed.
• Web worker. A second thread for scripts, which keeps heavy work off the main thread. See web workers.
• WebP. An image format from Google with lossy and lossless modes. See image formats.
• WOFF2. The compressed file format for web fonts that all modern browsers support. See web fonts and speed.

Check your own page

Run the free audit and read the report with this page open. Look up each metric and finding you do not know. Then open the linked guide for the ones that fail, starting with the Core Web Vitals.

Sources

• The MDN glossary: entries for Brotli, latency, bandwidth, origin, jank, lossy compression and network throttling.
• DOMContentLoaded on MDN.
• The guides linked from each entry, which carry their own sources.