Asynchronous Loading

Asynchronous loading is a technique where web page elements, scripts, or resources load independently without blocking the rendering of other page content. Scripts marked as asynchronous download in parallel with page parsing and execute as soon as they're available, without waiting for or delaying other resources.

Meaning & Context

In contrast to synchronous loading, where each resource must fully load before the next begins, asynchronous loading allows multiple resources to download simultaneously. For JavaScript files, the async attribute tells browsers to continue parsing HTML while the script downloads, then execute the script immediately upon completion. This prevents long-running scripts from blocking page rendering and improves perceived performance.

Why It Matters

Asynchronous loading is critical for A/B testing implementations because most testing tools load asynchronously to avoid blocking page rendering. While this improves overall page performance, it creates the potential for flickering since page content may render before test variations are applied. Understanding this trade-off helps optimize test implementations to balance performance with experience quality.

Example

An A/B testing platform loads its JavaScript library asynchronously, allowing the page to render immediately while the testing code downloads. The page displays in 1.5 seconds instead of 3 seconds with synchronous loading, but requires an anti-flickering script to prevent users from seeing content changes when the test code executes after the initial render.