Why is it important to collect metrics on the client?

Because everything that happens in the user's browser is invisible to the server. And that is exactly where the real problems hide.

Here's why it matters:

#1. Real load time

The server can be fast, but if the user has a weak device, a slow connection, or broken resources, the page lags. Only the client shows this.

#2. Errors the server never sees

A JS crash, a broken button, a rendering bug, none of it reaches the server. The user just sees a blank screen. Without client-side metrics, you never find out.

#3. User perception is not the same as technical metrics

Time to first byte (TTFB) is fast. But the interface appears after 5 seconds? The user is already gone. The client captures perceived performance, how it feels, not how it performs from the hardware's point of view.

#4. Devices, browsers, networks

A user might be on an old Android phone or in Safari 12. The server doesn't know that. The client does. Client-side metrics show exactly who is really struggling.

#5. Web Vitals

Metrics like Largest Contentful Paint, First Input Delay, and Cumulative Layout Shift are all measured only on the client, because they reflect the visual and interactive experience.

#Conclusion

The client is the truth. The server is theory. Client-side metrics show how the application feels on the other side of the screen. Without them, you're working blind.