Performance Trade-Offs

Overview

Convert's tracking script presents a fundamental choice between user experience consistency (no flicker) and Core Web Vitals optimization (better page speed scores).

Script Metrics

Script size increases with the number of active experiences, locations, audiences, and goals in the project.

Core Web Vitals Impact

Largest Contentful Paint (LCP)

• Synchronous: Moderate to high impact (script is render-blocking)
• Asynchronous: Minimal impact

First Input Delay (FID)

• Synchronous: Moderate impact (~200ms processing)
• Asynchronous: Minimal impact

Cumulative Layout Shift (CLS)

• Both modes: Low to moderate (depends on experiment design)
• Mitigation: Use anti-flicker techniques and reserve content space for elements that change

When to Choose Synchronous Loading

• User experience consistency is critical
• A/B testing accuracy is paramount
• Flicker effects are unacceptable
• Core Web Vitals are not a primary SEO concern

<script src="//cdn-4.convertexperiments.com/v1/js/[account_id]-[project_id].js"></script>

When to Choose Asynchronous Loading

• Core Web Vitals optimization is critical for SEO
• Page speed is prioritized over perfect UX consistency
• Slight flicker is acceptable for better performance

See the Anti-Flicker Async Loading recipe for the complete async snippet with built-in flicker protection.