Speed Index
Speed Index is a lab-based performance metric that measures how quickly the visual content of a page is populated during the page load. A lower score is better, as it indicates that the majority of the content in the visible viewport was rendered quickly. It is a key metric reported by tools like Lighthouse and WebPageTest to capture the perceptual speed of the loading experience.
Related web performance terms
- Search Engine Optimization (SEO)
SEO is the process of improving a website's visibility in search engine results pages (like Google). The goal is to rank higher than competitors for relevant search queries to attract more organic (free) traffic.
- Server Response Time
Server Response Time is a component of Time to First Byte that measures how long the server takes to process a request and start sending back a response, excluding network latency.
- Server Timing
Server Timing is a browser API that allows the backend to communicate performance metrics about its own internal processes (e.g., database query time, time to render a component) to the browser via HTTP headers.
- Server-Side A/B Testing
This is a performance-friendly method for running A/B tests. Unlike Client-Side A/B Testing, the decision of which variation to show a user is made on the Origin Server.