Measuring Core Web Vitals with Sentry

Core Web Vitals: Beyond Abstract Metrics to Real User Experience

Measuring website performance is crucial, and the Google-defined Core Web Vitals offer a smart approach. While optimizing requests, caching, and file sizes are important, Core Web Vitals directly address user experience: load time, interactivity, and visual stability. They measure what truly matters – how long users wait for content, when they can interact, and whether the page jumps around unexpectedly.

Many tools measure these metrics, but Sentry's approach stands out by leveraging real user data ("field data"). Unlike synthetic lab data from tools like Google Chrome Lighthouse (which controls environment and network speed), Sentry gathers aggregate data reflecting actual user experiences across various network speeds, browsers, devices, and regions. This provides a comprehensive understanding of performance issues in the real world – their frequency, causes, and connections to other behaviors.

Sentry's power lies in its ability to break down transactions into key metrics, revealing the impact on users. For instance, it might highlight that 42% of transactions experience input delays exceeding 301ms, correlating this with other performance problems. This is vital because even a fast app for some users might be painfully slow for others. Visualizing this real user data helps prioritize and address performance bottlenecks effectively.

The importance of context is key. A five-second First Paint might be acceptable on a settings page, but three seconds on a checkout page is unacceptable. The same metric can have vastly different implications depending on the page and user flow.

The combination of real-time user experience data and the visualization of metric changes over time is transformative. While Lighthouse scores are helpful, they lack the accuracy of user-specific data from your own application.

Sentry's Performance section tracks transaction improvements and regressions over time. Internal use of Sentry by its own engineers demonstrates its effectiveness. By analyzing "Most Regressed Transactions," developers can pinpoint problematic transactions and quickly identify the root cause, as illustrated by Tony Xiao's experience.

This evolution in front-end development tools is exciting. The improved understanding of performance issues, coupled with evolving language, tools, and techniques, marks a significant shift in the industry. The focus is shifting from abstract metrics to a more user-centric approach, which is something to celebrate.

1. The author works at Sentry, but this is not a sponsored post. This commentary is based on personal observations following a colleague's post on Core Web Vitals. ↩️

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