Performance Optimization Glossary

Key Terminology

Improving website speed is one of the best ways to improve user experience and streamline the path to sales. Understanding the terminology is a great first step to learn performance optimization and the many interrelated techniques to improve load time. This glossary of key terms can help make sense of all the jargon and how the many pieces of the website load time puzzle fit together.

Above The Fold

Sometimes also called "above the scroll", above the fold is a concept that originated in print design. For websites, above the fold refers to the content of a page initially visible in the browser window, before scrolling. Prioritizing above the fold content to load first and fast can go a long way to improve the perception of speed.

Bounce Rate

A common measurement of traffic retention, bounce rate refers to the percentage of visits that only view one page. Bounce rate can be affected by website speed if users abandon the page before loading is complete. Along with other metrics like pages per visit and session duration, bounce rate can help formulate a picture of general user engagement.


A content delivery/distribution network is a form of web hosting that improves loading efficiency by delivering files from a large network of geographically-distributed servers. CDNs are typically used as a supplement to existing web hosting to store and distribute file resources that change infrequently, like images, CSS and JavaScript. For users distributed over a wide geographic area, a CDN can improve performance by loading some files from closer web servers.

Minify Code

Formatting website code with spaces, line breaks, indentation and comments helps to keep code organized and easy to read, but extra spacing doesn't benefit web browsers and increases file size. To make code files as small as possible, 'minifying' files like CSS and JavaScript by eliminating all superfluous spaces helps reduce page weight for faster loading.


HTTP/2 is the latest version of the networking protocol through which web servers communicate with web browsers and transfer files. One of the benefits of using HTTP/2 for website speed is the capacity to transfer multiple files simultaneously, rather than just one or a few at a time. Particularly for pages that load a lot of file resources, setting up the web server with HTTP/2 is an important performance upgrade.

HTTP Compression

HTTP compression is data compression applied to files by the web server before being transferred to the user's web browser, decompressed and loaded. Reducing the quantity of data that has to be transferred over the internet is an important factor in maximizing website speed so properly configuring HTTP compression is a must for complete website optimization.

Image Compression

Image compression is the process of applying data compression to images for reduced file size and is one of the most important steps for complete image optimization. Image compression can be used to reduce file size while preserving image quality or with an acceptable loss of visual quality to reduce file size even more. The degree of compression possible for any given image depends on the file format, the desired visual quality and the content of the image, like photographic imagery vs graphics composed of lines and shapes.

Image Sprite

An image sprite is an image file that contains multiple images. Each individual image is then shown as needed, typically using background image techniques in CSS. Image sprites are most often used to group similar graphics like icons. Consolidating many small images into a few larger images reduces HTTP requests and can improve file compression.

Landing Page

Landing pages are the pages of a website that visitors see first. The homepage is often a landing page, but if a user arrives from a search results page they may land on an interior page like a category page or a specific product page. A marketing campaign might also drive traffic to purpose-built landing pages. These kinds of pages often warrant the most aggressive performance optimization to maximize traffic retention.

Lazy Loading

Lazy loading refers to deferring or delaying the loading of selected resources to the end of the loading process or until those resources are specifically needed. This is often used for images - only loading each image when the user has scrolled to that point on the page. For users that may not see the entire page, lazy loading prevents images that are never seen from loading at all.

Load Time

Load time is the time elapsed between a user action to navigate to a page and when that page is loaded, visible and ready for user interaction. One of the primary measurements of website's speed, minimizing load time is an important factor in everything from search engine optimization and traffic retention to user engagement and sales conversion.

Page Weight

Load time is often the focus of performance measurements and optimization strategies, but page weight is also important because load time can vary depending on internet connection speeds. Page weight is the total data of all the file resources needed to load a page, like HTML, CSS, JavaScript and images as well as files loaded from third-party sources like font services and external plugins.

Render Blocking

Depending on the way HTML is structured, some file resources like CSS and JavaScript can momentarily delay the display of content while the page is loading. Excessive render blocking files can slow page loading and influence the perception of speed if content is hidden while the page completes the loading process.

Server Response Time

Response time is a basic measurement of web server speed. When a user navigates to a website, the web browser connects to the server and beings the loading process by requesting the files needed to view the page. Response time is the time it takes for the web server to answer that request and start transferring data. The faster the server responds, the sooner the loading process begins.

Speed Test

Also called a performance test or audit, a speed test is an online tool that analyzes website loading performance and returns a report of areas that are already optimized and areas that need improvement. The relative importance of the factors that influence load time are a little different with each test, so running more than one test is recommended.

Some of the most popular performance testing tools: