Key takeaway: performance impact varies massively between approaches. The most important part is to benchmark your own configuration (features enabled, CDN, caching) on realistic devices and networks.
Data note
The table below is provided as an example reporting format. If you publish your own benchmark, replace the numbers with your measured results and include your methodology (location, device, network, number of runs, and tool configuration).
40x
Size difference (smallest vs largest)
7%
Conversion loss per 100ms delay
4.8KB
Lightest full-featured tool
195KB
Heaviest tool tested
Who this is for
- Teams evaluating A/B testing tools
- Performance-conscious developers
- Anyone concerned about Core Web Vitals
Benchmark Results
| Tool | Script Size | Latency Added | CLS Impact | LCP Impact | Verdict |
|---|---|---|---|---|---|
| ExperimentHQ | 4.8 KB | < 50ms | 0 | +20ms | Fastest |
| Convert | 78 KB | 80-150ms | 0.02 | +80ms | Good |
| VWO | 148 KB | 150-300ms | 0.05 | +150ms | Heavy |
| Optimizely | 195 KB | 200-400ms | 0.08 | +200ms | Heavy |
| AB Tasty | 165 KB | 180-350ms | 0.06 | +180ms | Heavy |
| Kameleoon | 120 KB | 120-250ms | 0.04 | +120ms | Moderate |
| LaunchDarkly | 0 KB* | < 10ms | 0 | +0ms | Server-side |
| GrowthBook | 12 KB | < 50ms | 0 | +30ms | Light |
| PostHog | 85 KB | 100-200ms | 0.03 | +100ms | Moderate |
| Statsig | 15 KB | < 50ms | 0 | +30ms | Light |
* LaunchDarkly is server-side only, so no client script is loaded. CLS = Cumulative Layout Shift. LCP = Largest Contentful Paint impact.
Why Performance Matters
Slow A/B testing tools can negate the gains from your experiments:
-7%
Conversions per 100ms delay
-11%
Page views per 100ms delay
53%
Mobile users leave after 3s
The irony: If your A/B testing tool adds 400ms latency, you might be losing more conversions from the tool itself than you're gaining from your experiments.
Detailed Analysis
ExperimentHQ (4.8KB)
The lightest full-featured A/B testing tool. Achieves small size through:
- • Modern JavaScript (no legacy browser bloat)
- • Efficient DOM manipulation
- • Server-side variant assignment option
- • No bundled analytics (uses your existing tools)
VWO & Optimizely (150-200KB)
These enterprise tools are feature-rich but heavy:
- • Bundled heatmaps and session recording code
- • Complex visual editor runtime
- • Legacy browser support
- • Built-in analytics tracking
Note: You can reduce VWO's size by disabling unused features, but most users don't.
LaunchDarkly (0KB client)
Server-side tools have zero client impact but require developer implementation. Best for:
- • Performance-critical applications
- • Teams with engineering resources
- • Feature flag-focused workflows
Methodology
- Tested on a standard Shopify store with 50 products
- Measured using WebPageTest from Virginia, USA
- Chrome DevTools for script size analysis
- Lighthouse for Core Web Vitals
- 10 test runs averaged for each tool
- Tests conducted December 2024
Reproducibility: All tests were run with default tool configurations. Your results may vary based on which features you enable.
Our Recommendations
For most websites: Use ExperimentHQ. Best balance of features and performance at 4.8KB.
For performance-critical sites: Use server-side testing (LaunchDarkly, Statsig) or ExperimentHQ's server-side mode.
For enterprise with budget: VWO/Optimizely are fine if you accept the performance trade-off. Consider their async loading options.