TL;DR
Key finding: Heavy A/B testing tools (100KB+) add 120-240ms to LCP, 35-75ms to FID, and cause 0.05-0.12 CLS. This can push "Good" Core Web Vitals into "Needs Improvement," hurting SEO rankings. Lightweight tools (<10KB) like ExperimentHQ add <10ms impact—negligible.
Data note
The table below is a reporting template. If you publish a benchmark, replace numbers with your measured results and include device, network, location, run count, and tool configuration.
Core Web Vitals Impact (Median)
| Tool | LCP Impact | FID Impact | CLS Impact | Verdict |
|---|---|---|---|---|
| ExperimentHQ | +5ms | +2ms | 0 | Minimal |
| PostHog | +25ms | +8ms | 0.001 | Low |
| Convert | +120ms | +35ms | 0.05 | Medium |
| VWO | +180ms | +55ms | 0.08 | High |
| Optimizely | +240ms | +75ms | 0.12 | Very High |
Core Web Vitals Thresholds
| Metric | Good | Needs Improvement | Poor |
|---|---|---|---|
| LCP (Largest Contentful Paint) | < 2.5s | 2.5-4s | > 4s |
| FID (First Input Delay) | < 100ms | 100-300ms | > 300ms |
| CLS (Cumulative Layout Shift) | < 0.1 | 0.1-0.25 | > 0.25 |
SEO Consequences
Core Web Vitals are a Google ranking factor. Poor CWV can:
- • Lower search rankings — especially on mobile
- • Lose "Good" badge — in Search Console
- • Hurt user experience — slow pages = higher bounce rates
The catch-22: Adding a heavy A/B testing tool to improve conversions can hurt SEO, reducing traffic.
Choose Performance-First Tools
ExperimentHQ is designed for Core Web Vitals. <5KB script, minimal impact on LCP/FID/CLS.