JavaScript setup to improve FCP
Sites created after January 2026 have Adaptive JS and Tail JS enhancements enabled automatically during setup.
For sites created before January 2026, request enablement from the Support team.
Adaptive JavaScript
This reduces a site's JavaScript bundle size by approximately 10-20%. This means less code needs to be downloaded, parsed, and executed before content can be displayed.
Our testing shows an average improvement of approximately 0.2 seconds in First Contentful Paint times when this configuration is enabled.
Outcomes:
- Optimizes which JavaScript code is delivered to users
- Removes unnecessary code paths based on usage patterns
- Maintains all functionality while reducing file size
Tail JavaScript
Tail JavaScript changes how a site's scripts are loaded by moving non-essential JavaScript execution to the end of the page load process. This allows the browser to render visible content faster, without waiting for all scripts to be processed.
Expert testing shows an improvement of approximately 0.2+ seconds in First Contentful Paint when this configuration is enabled.
Outcomes:
- Defers JavaScript execution to after initial content rendering
- Reduces render-blocking behavior of scripts
- Prioritizes visual content over background functionality
Combined approach
When both JavaScript configurations are applied, the improvements are more substantial. These include FCP improvements of 0.2-1.0 seconds in production environments, and are most noticeable on pages with significant JavaScript requirements.