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 multiple synchronous downloads. Script size starts at a base size and scales with each experiment.

|               | Uncompressed | Compressed |
| ------------- | ------------ | ---------- |
| Base script   | 79KB         | 20KB       |
| Per-flag size | ~1KB         | ~100B      |

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

{% callout type="note" heading="Custom code impact on flag size" %}
Custom code increases a flag's code size by the size of the custom code itself.
{% /callout %}

## Caching

Web Experiment uses two caching layers: CDN and browser. The two layers make script delivery to your site more reliable.

### CDN cache

Amplitude caches the Web Experiment script on a CDN. When a user requests the script, the browser loads the script 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 for up to one hour while the script reloads. If the origin returns an error, the CDN serves a stale response for the maximum 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 a network request for 60 seconds, or for the maximum time if the server returns an error. The browser cache serves the script from memory (0ms latency) when 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 using information available synchronously in the browser. Evaluation is CPU bound and usually takes less than 1ms to apply variant actions.

## Ad blockers

Amplitude can't identify users with ad blocking software enabled. Those users don't log assignment events or impressions, and don't experience the experiment.