Visually Complete vs Speed Index
What is the best metric to consider in measuring your customers’ actual experience with your applications? As it turns out, users are primarily focused on how long it takes for visual page elements above the fold to render completely.
Visually complete is a user experience metric that identifies the moment in time when users perceive that all the visual elements of a page have completely loaded. Because this is a measure of user perception, it is an ideal measure of user experience. Visually complete metrics are typically shorter in duration than comparable metrics, for example, page load time, because users perceive complete page load before 100% of background page elements have loaded. As you can see in the example below. The Visually complete milestone is reached before the page has loaded completely.
Speed index is a performance metric that measures how quickly a page renders visual elements, from the user’s perspective. This is a rate-of-speed metric that is closely related to Visually complete, which is a moment-in-time measure. A lower Speed index is preferred as it means that large parts of a page render quickly. Speed index, which was introduced by webpagetest.org, takes the “visually complete” concept one step further. Pages that render large visible elements earlier (see example below) receive better scores than pages that render elements slowly (see example below on the right) even when those pages have an identical Visually complete measure.