Performance Optimization for A/B Test Scripts

Overview

A/B testing is powerful, but poorly optimized test scripts can slow down your website, hurt user experience, and even skew experiment results. Good performance isn’t optional — it is essential for clean, reliable experiments. Use the following practical tips when building A/B tests.

1. Keep DOM Manipulation Light

A/B tests commonly modify elements on the page, but heavy or repeated DOM operations can cause rendering delays or layout shifts.

Tips:

• Use classList.add() instead of rewriting HTML.
• Batch DOM changes — avoid modifying layout inside loops.

• Cache selectors:

  const btn = document.querySelector('.cta');
  

  instead of querying repeatedly.

2. Avoid Flash of Original Content (FOOC)

If users see the original content for a moment before the variant is applied, your test loses accuracy and feels glitchy.

Fixes:

• Use CSS pre-hiding until the script finishes.

• Add:

  .ab-hide { opacity: 0; }
  

• Apply early:

  document.documentElement.classList.add('ab-hide');
  

• Remove after the variant is applied.

3. Don’t Block Page Loading

Blocking <script> tags delay rendering.

Better:

• Use async or defer to load scripts.
• Use a single bundled experiment file.
• Keep script size under 20–30 KB.

4. Use Efficient CSS Instead of JavaScript

If something can be done with CSS — choose CSS. It’s cheaper and faster.

Examples:

• Hiding elements
• Changing colors
• Adjusting spacing/margins

CSS avoids JS execution and reduces repaints.

5. Reduce Polling & Interval Checks

Heavy polling like:

setInterval(() => {...}, 50);

can be expensive.

Optimizations:

• Use MutationObserver.
• Use one global timer instead of many.
• Always stop polling once done.

Example:

const observer = new MutationObserver(() => {
  const el = document.querySelector('.product-box');
  if (el) {
    observer.disconnect();
    runExperiment();
  }
});

observer.observe(document.body, { childList: true, subtree: true });

6. Keep Logic Inside the Test Minimal

Avoid turning test scripts into complex mini-apps.

Avoid:

• Complex logic
• Heavy loops
• Multiple API requests

Keep variants simple and lightweight.

7. Avoid Layout Thrashing

Mixing read/write layout operations triggers forced reflows.

How to avoid:

• Read layout values once and store them.
• Group reads separately from writes.
• Avoid mixing inside loops.

8. Use Delegated Event Listeners

Instead of adding many event listeners:

document.body.addEventListener('click', (e) => {
  if (e.target.matches('.cta-btn')) {
    // handle click
  }
});

This improves performance and reduces memory leaks.

9. Remove Test Code After Experiment Ends

Old experiment scripts often remain in production unnecessarily.

Always:

• Remove inactive test code
• Remove unused test CSS
• Remove external resources

10. Prioritize Core Web Vitals

A/B tests affect metrics such as:

• CLS – layout shift
• LCP – hero image/content loading
• FID / INP – interaction delay

Best practices:

• Don’t move large elements after load.
• Don’t delay hero images.
• Avoid last-second DOM changes.

11. Cache Heavy Data

If your test needs product or API data — cache it.

Use:

• localStorage
• sessionStorage

This avoids repeated network calls.

12. Test Performance Before Launch

Before going live:

• Run Lighthouse
• Check console errors
• Test on low-end devices
• Ensure no FOOC
• Verify no layout jumps