Glossary of Web Speed Terms
If you ever wondered what a given term meant on your page speed report, here is the beginning of a long list of terms, explained and defined, so that you will no longer have to guess what they mean.
CPU (central processing unit) is the computing part of the computer, also called the "processor”. The CPU is made up of the control unit, which executes the instructions, and the ALU, which performs the calculations and logical operations. Nowadays, the CPUs of almost all computers are contained on a single microprocessor chip.
CSS files are cascading style sheets which describe how HTML elements (e.g., color, fonts, layouts) should appear and be presented on webpages. They are also responsible for how they should adapt when viewed on different devices. If HTML is the skeleton, CSS files are the looks of the website.
Critical Request Chains
Critical Request Chains are a series of dependent network requests important for page rendering. The greater the length of the chains and the larger the download sizes, the more significant the impact on page load performance.
DOM (document object model) is a programming interface for HTML and XML documents. It represents the page so that programs can change the document structure, style, and content. In other words, the DOM represents a document with a tree, in which each branch ends in a node, and each node contains objects. Thus, with DOM methods one can change the structure, style or content of a document.
ETag (entity tag) is a feature of the HTTP protocol, which speeds up requests by ensuring that the data stored in a web cache is up-to-date. The ETag is an identifier assigned to a data resource in a server, and if that resource is ever updated at the server, the ETag changes too.
FCP (First Contentful Paint) measures the time from a user’s navigation to the time when the browser renders the first bit of content from the DOM. In other words, FCP marks the time at which the first text or image is painted to the user.
FMP (First Meaningful Paint) measures when the primary content of a page is visible. The raw score for FMP is the time in seconds between the user initiating the page load and the page rendering the primary above-the-fold content. FMP essentially shows the timing of the paint after which the biggest above-the-fold layout change happens.
HTML (Hypertext Markup Language) is an essential language which is used in order to specify content for a web page. It consists of tags and attributes that tell the browser what content the web page contains, e.g. things like images and text are all specified with HTML.
HTTP (HyperText Transfer Protocol) is how data is transferred from a computer server to a web browser. It is the standard protocol for transferring hypertext documents on the World Wide Web. HTTP defines commands and services used for transmitting webpage data. Basically, it is the underlying protocol which allows users to exchange information.
Redirect Chain happens when a page on your website redirects to a page that’s already redirected. A redirected page is a page that sends the search engine or the user who requested one page to another webpage, which is also relevant. Redirect chains can possibly hurt your website loading speed.
SLA (Service Level Agreement) is a contract between the service provider and the user that specifies the level of service expected during its term, for example for cloud storage, hosting, or CDN services. SLAs can specify bandwidth availability, response times for routine and ad hoc queries, response time for problem resolution (network down, machine failure, etc.).
Speed Index shows how quickly the contents of a page are visibly populated. It measures how quickly content is visually displayed during page load. Speed Index is expressed and measured in milliseconds and it is dependent on the size of the view port.
TTFB (time to first byte) is the number of milliseconds it takes for a browser to receive the first byte of the response from the user’s web server. It appears when a user selects the given website on Google and then their web browser requests information (i.e. bytes) from a server.
TTI (Time to Interactive) is the amount of time it takes for the page to become fully interactive. In this sense, a page is considered fully interactive when the page displays useful content, which is measured by the FCP, when event handlers are registered for most visible page elements, and when the page responds to user interactions within 50 milliseconds.
WPO (Web Performance Optimization) is the process, methodology and different techniques to measure, benchmark and improve app performance. In this sense, performance refers to the speed at which application pages are rendered on the end-user’s browser. WPO is the way of tuning a web application for faster and smoother page loads, and an overall better user experience.
Watch out for more terms coming soon!