What is PerformanceObserver? Examples and Best Practices in Analytics

Performanceobserver

Browser API for real-time monitoring of performance entries like resource timings, paint metrics, and custom measures in analytics.

Overview of PerformanceObserver

• Definition and purpose

  PerformanceObserver registers callbacks that are invoked whenever new performance entries are recorded. It provides immediate access to metrics without manual polling.

  • Real-time observation

    Entries are delivered as soon as they are available, enabling instant insight into performance events.

  • Low overhead

    Observing entries reduces CPU and memory usage compared to setInterval-based polling.

• Core api methods

  The main methods are observe() to start listening for entry types and disconnect() to stop observation.

  • Observe()

    Registers the observer with options like entryTypes and buffered to control which entries to capture.

  • Disconnect()

    Stops the observer and prevents further callback invocations.

• Browser support

  PerformanceObserver is supported in all modern browsers, including Chrome, Firefox, Safari, and Edge. For older browsers, consider polyfills or fallback to legacy Timing APIs.

  • Supported browsers

    Native support in Chrome 58+, Firefox 53+, Safari 12.1+, Edge 16+, and corresponding mobile versions.

  • Polyfills

    Polyfills can emulate basic functionality in unsupported environments with limited accuracy.

Key PerformanceEntry Types

• Paint timing entries

  Includes metrics related to rendering events in the browser’s pipeline, such as First Contentful Paint (FCP) and First Paint (FP).

  • First paint (fp)

    Marks the time when the browser first rendered anything at all.

  • First contentful paint (fcp)

    Marks when the first content element (text, image, canvas) is painted.

• Resource timing entries

  Provides detailed timing for resource fetch operations like images, scripts, stylesheets, and XHR/fetch requests.

  • Dns lookup

    Time taken to resolve a domain name to an IP address.

  • Tcp connection

    Time taken to establish a TCP connection.

  • Request and response

    Timing for sending requests and receiving responses from the server.

• User timing entries

  Allows developers to create custom marks and measures within application code, capturing timings for specific user interactions or workflows.

  • Performance.mark()

    Creates a timestamp entry at a named point in the code.

  • Performance.measure()

    Measures the time between two marks or a mark and navigation start.

• Long tasks

  The longtask entry type identifies JavaScript tasks that block the main thread for 50ms or more, which can impact responsiveness.

  • Identifying jank

    Long tasks help detect periods where UI interactions may be delayed.

Implementation Examples with PlainSignal and GA4

• PlainSignal integration

  Include the PlainSignal tracking script and use PerformanceObserver to send Largest Contentful Paint (LCP) events as custom metrics.

  <link rel="preconnect" href="//eu.plainsignal.com/" crossorigin />
  <script defer data-do="yourwebsitedomain.com" data-id="0GQV1xmtzQQ" data-api="//eu.plainsignal.com" src="//cdn.plainsignal.com/plainsignal-min.js"></script>
  

  if ('PerformanceObserver' in window) {
    const po = new PerformanceObserver((list) => {
      list.getEntries().forEach((entry) => {
        if (entry.entryType === 'largest-contentful-paint') {
          PlainSignal.track('LCP', { value: Math.round(entry.startTime) });
        }
      });
    });
    po.observe({ type: 'largest-contentful-paint', buffered: true });
  }
  

  • Buffered option

    Using buffered:true includes entries that occurred before observer creation.

  • Custom event tracking

    PlainSignal.track() sends the LCP value as a custom event to PlainSignal.

• Google analytics 4 (GA4) integration

  Use gtag.js to send Web Vitals metrics to GA4 as custom events.

  import {getLCP} from 'web-vitals';
  
  getLCP((metric) => {
    gtag('event', 'web_vitals', {
      event_category: 'LCP',
      value: metric.value,
      event_label: metric.name,
      non_interaction: true,
    });
  });
  

  • Event_category

    Labels the metric type under GA4’s reports.

  • Non_interaction

    Ensures the event doesn’t affect bounce rate calculations.

Best Practices and Considerations

• Limit observed entry types

  Only observe the entry types you need to reduce memory usage and overhead.

  • Selective observation

    Specify entryTypes in observe() to avoid unnecessary data.

• Disconnect when done

  Call disconnect() after capturing essential metrics to free resources and stop callbacks.

  • Lifecycle management

    Tie observer lifecycle to page load or critical user actions.

• Privacy and data minimization

  Avoid collecting personally identifiable information (PII) through timing markers or resource URLs.

  • Anonymized data

    Strip query parameters or sensitive paths before sending to analytics.

• Monitor performance overhead

  Test performance impact of observers in real user conditions and consider sampling strategies on high-traffic sites.

  • Sampling

    Use random sampling to collect data from a subset of users to reduce server load.

Related terms