Sentry Web Vitals: Unpacking and Resolving LCP Metric Detection Challenges
Tags: sentry, frontend, javascript, webdev
Published: 2023-10-04 14:58:39

For those unacquainted, Sentry isn't just a tool for error monitoring. It's a robust platform that also allows for the measurement and analysis of web vitals metrics. These metrics provide tangible insights into the performance of web applications from the end-user's perspective. As we move deeper into the digital age, understanding and optimizing for user-centric metrics becomes paramount to delivering a top-notch user experience. However, while these metrics are designed to reflect real-world user interactions, they employ heuristics in their measurements. And inherently, heuristic approaches, though efficient, aren't always flawless. This brings us to the challenges and nuances of the Largest Contentful Paint (LCP) metric detection in Sentry, as we detail below.

**The Background**:
Recently, we encountered an issue with the LCP metric detection in Sentry. If you're not familiar with the LCP, it's a core web vital metric that represents the point in the page load timeline when the largest content element of a page is likely visible to users. Having this metric is crucial in understanding how well our application is performing from a user's perspective.

**The Problem**: 
The LCP metric in Sentry wasn't behaving as expected. After diving deep into the operational aspects of this metric within Sentry, we identified the root cause. In our application, we remove the loading splash screen only after all resources have loaded and we receive the server response with some data. The Sentry transaction, however, is dispatched after the "load" event on the page, combined with specific Sentry processes and an `idleTimeout` set to the default 1000ms.

**The Solution**: 
To give the content enough time to render on the page, we opted to adjust the `idleTimeout` parameter. This modification ensures that the "pageload" transaction in Sentry is dispatched later, allowing the primary content enough time to become visible.

Additionally, through experimentation, we discovered that older versions of Sentry only collect the LCP metric after a user interaction, particularly a click on an interactive element. This means, after our changes, the LCP metric will be captured only if a user interacts with an element within the `idleTimeout` duration post page load.

**What’s Next?**: 
The good news is that the latest version of Sentry rectifies this limitation. Upon updating Sentry, the LCP metric will be automatically recorded at the end of the “pageload” transaction. Following this, the metric will be noted even if the user hasn't interacted with the page.

In conclusion, while web vitals metrics offer invaluable insights, understanding their nuances and ensuring accurate setups can greatly improve the user experience. If you've been grappling with Sentry's LCP metric, consider revisiting your `idleTimeout` settings and updating to the latest version.

With the new version of Sentry, the LCP metric is recorded automatically at the end of the "pageload" transaction. However, in our specific scenario, adjusting the `idleTimeout` remains imperative. Without it, the transaction would conclude prematurely. This is primarily because, in our application design, we remove the loading splash screen only after receiving pertinent data from the backend. This makes the `idleTimeout` adjustment a necessary step to ensure the LCP metric is captured accurately and reflects the true user experience.

For a deeper dive into the technical specifics of this issue, consult the related [Sentry GitHub issue](https://github.com/getsentry/sentry-javascript/issues/6742#issuecomment-1739351303).