Performance Assertions

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Performance assertions let you verify Core Web Vitals and other key performance metrics for the current page.

How Capture Works

Performance data is navigation-based. Metrics are measured for the most recent page navigation:

Automatic measurement: KaneAI uses the web-vitals library to capture metrics
Per-navigation scope: Metrics reflect the last full page load. If you navigate to a new page, the metrics reset for that navigation
Point-in-time snapshot: When a performance checkpoint triggers, KaneAI captures the current metrics at that moment

What This Means for Your Tests

Performance metrics describe the last navigation. If you navigate to Page A then Page B, the metrics reflect Page B
Place performance assertions after the page you want to measure has fully loaded
Use a wait step if the page needs time to settle before measuring

Available Metrics

Metric	What It Measures	Good Threshold	Learn More
LCP	Largest Contentful Paint: when the largest visible element finishes rendering	< 2,500ms	web.dev/lcp
CLS	Cumulative Layout Shift: visual stability, how much the page layout shifts	< 0.1	web.dev/cls
INP	Interaction to Next Paint: responsiveness to user input	< 200ms	web.dev/inp
FCP	First Contentful Paint: when the first content appears on screen	< 1,800ms	web.dev/fcp
TTFB	Time to First Byte: server response time	< 800ms	web.dev/ttfb

Note: Not all metrics are available for every page. INP requires user interaction to trigger. Some metrics may be null if the browser hasn’t measured them yet.

Example Assertions

Assert: page LCP is under 2500ms
Assert: CLS is below 0.1
Assert: TTFB is under 800ms
Assert: FCP is less than 1800ms
Assert: page performance meets Core Web Vitals thresholds

Example Extractions

Store the page LCP value
Extract all web vitals metrics
Store the TTFB for this page

Example If/Else

If LCP is under 2500ms then continue, else report performance issue

Tips

Wait for load: Place a wait step before performance assertions to ensure the page has fully loaded and metrics are available
Navigate first: Metrics are per-navigation. Make sure you’ve navigated to the target page before asserting
Not all metrics are instant: CLS accumulates over time, INP requires interaction. LCP and FCP are typically available after the page visually completes loading

​How Capture Works

​What This Means for Your Tests

​Available Metrics

​Example Assertions

​Example Extractions

​Example If/Else

​Tips