Does Split Testing Slow Down My Site?

Yes, slightly. Nelio A/B Testing is designed to minimize its impact on page loading times, and most of its operations run while the page is loading.

Starting with Nelio A/B Testing 8.3, the size of the public tracking script bundle was reduced by approximately 65%, further reducing its impact on page loads and Core Web Vitals.

Why Is There Is a Small Performance Impact?

When a visitor lands on your site while tests are running, Nelio A/B Testing performs a few quick checks:

1. Whether the current page is affected by a running test.
2. Whether the visitor participates in the test.
3. Which variant they should see.

These checks are lightweight and have minimal impact on page loading.

The step that may introduce a more noticeable delay is loading alternative content, depending on the variant delivery method being used.

JavaScript Redirection (URL Parameters)

By default, if a visitor initially loads a URL but should see alternative content, Nelio A/B Testing may perform a JavaScript redirection to a URL containing the nab query argument.

Because the appropriate page then needs to be loaded, this can introduce a small additional delay.

To reduce these redirections when visitors navigate through your site, you can enable:

Settings → Behavior → Add testing query arg nab to all URLs to speed up page loading times when browsing your site

This allows internal links to include the necessary testing information in advance, so the appropriate alternative content can be requested directly when possible.

Dynamic Caching (Cookie-Based Variant Loading)

If your hosting or caching setup supports dynamic caching based on cookies, Nelio A/B Testing can load the appropriate alternative content without using the JavaScript redirection.

This can provide a smoother loading experience, but it requires a compatible caching setup and additional configuration.

See this separate guide for enabling dynamic caching for more information.

What About CSS & Text and JavaScript Tests?

CSS & Text tests and JavaScript tests don’t use redirects to load their variants. Their changes are applied directly on the page, avoiding the additional page load required by redirection-based tests.

Summary

• Nelio A/B Testing is designed to keep its performance impact minimal.
• Version 8.3 reduced the public tracking script bundle size by approximately 65%.
• Redirection-based variant loading may introduce a small additional delay.
• Adding the nab query argument to internal links can reduce the need for redirections while browsing.
• Dynamic caching can avoid these redirections when supported and properly configured.
• CSS & Text and JavaScript tests apply their changes directly without redirection.