Web Experiment performance

Web Experiment intentionally minimizes its impact on page performance.

Script size

The Web Experiment script is dynamic, and includes all your experiment configurations to avoid making multiple synchronous downloads. This means that the script size starts with a base size, and scales with each experiment.

To avoid constantly increasing script sizes, deactivate or archive experiments when they're complete.

Caching

Web Experiment uses two layers of caching: CDN and Browser. This helps to provide more reliable script delivery to your site.

CDN cache

Amplitude caches the Web Experiment script on a CDN. When a user requests the script, their browser loads it from the CDN if another user loaded it in the same geographic area. The CDN cache has a max age of one minute, and serves stale content while the script reloads for up to one hour. The script serves a stale response if the origin returns an error for the maximum amount of time possible.

The cache control response header that configures CDN caching is:

max-age=60,stale-while-revalidate=3600,stale-if-error=31536000

Browser cache

The browser cache serves the Web Experiment script without making a network request for 60 seconds, or the maximum amount of time if the server returns an error. This caching layer serves the script from memory (0ms latency) if a user loads multiple pages on your site, or reloads the same page within a one minute window.

The cache control response header that configures browser caching is:

max-age=60,stale-while-revalidate=3600

Evaluation

Web Experiment evaluation runs locally with information available synchronously in the browser. As a result, evaluation is CPU bound and usually takes less than 1ms to evaluate and apply variant actions.

Ad blockers

Amplitude can't identify users who have ad blocking software enabled. As a result, those users don't log assignment events or impressions, and don't experience the experiment.