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The Server-Timing response header lets a server or intermediary report selected processing measurements for a request. To use it, instrument useful server-side steps, emit their metrics in the response, inspect them beside that request’s browser timing in DevTools, and—if you need automation—read them from navigation or resource entries in the Performance API. These measurements are diagnostic clues, not a complete explanation of page load time.
What Server-Timing tells you
A response can report named components such as cache lookup, database work, or application processing. For example:
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Server-Timing: cache;desc="Cache Read";dur=23.2, db;dur=53, app;dur=47.2Each metric can have a name, a duration (dur), a description (desc), or both duration and description. A response may include multiple metrics; their order is not significant. Keep names and descriptions short to limit response-header overhead. The values are measurements selected and supplied by the server or intermediary, not timings the browser has inferred. MDN’s Server-Timing header reference and the W3C Working Draft dated April 7, 2026 describe the format.
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The header is useful only when your application or intermediary measures relevant work and emits the results. Choose steps that help answer a real question about a request—for example, cache lookup, database operations, or application processing. The W3C draft illustrates names such as cache, db, and app; those are examples, not a prescribed set or evidence of typical performance.
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Decide which component owns each measurement and what the duration covers. A cache metric might describe an intermediary’s work, while an application metric might describe a selected portion of origin processing. The header does not automatically identify every cache layer or backend operation: it can report only what is measured and included.
Inspect Server-Timing in browser DevTools
- Open your browser’s developer tools and select the Network panel. In Chrome DevTools, this is the Network panel; presentation varies among browsers and browser versions.
- Reload the page or reproduce the request, then select the specific network request you want to investigate.
- Inspect the response headers for
Server-Timingand the selected request’s timing breakdown. Read the header values as server- or intermediary-reported components for that request, and the browser breakdown as client-observed request activity. - Compare the two views to form a next diagnostic question. For example, a large reported database duration may justify investigating database work, but it does not prove the database alone caused a user-visible delay.
Chrome documents its Network panel in Inspect network activity. The exact placement and labels of timing information can differ by browser and version. DevTools may also display a Server-Timing HTTP trailer, but Fetch cannot access HTTP trailers, as MDN notes.
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Read metrics programmatically with the Performance API
For automation or client-side diagnostics, server timing is exposed on resource timing entries and navigation timing entries. A PerformanceObserver can process both entry types:
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for (const entry of list.getEntries()) {
for (const metric of entry.serverTiming) {
console.log(metric.name, metric.description, metric.duration);
}
}
});
observer.observe({ type: "navigation", buffered: true });
observer.observe({ type: "resource", buffered: true });Each exposed metric provides name, description, and duration. The API reports what the response exposed; it does not create server measurements. You still need server-side instrumentation and header emission. There are no separate server-timing performance entries: inspect the serverTiming property on navigation or resource entries. See MDN’s Server timing API reference.
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Interpret server and browser timings without conflating them
Use the browser’s request timing to understand client-visible network activity, and Server-Timing to see which selected work the server or intermediary reports. They describe different scopes, so do not assume that server component durations add up to the browser-observed interval or to full page-load time. A reported duration is evidence about that measured component, not proof of root cause.
Server-Timing is not a way to send arbitrary structured data or to align a server startTime with the browser clock. The W3C’s Server Timing Explained says: “It is not the goal of this API to provide a mechanism which relies on clock synchronization (between client, server, or any intermediaries), like providing startTime.” Treat the measurements as component durations, not synchronized client/server timestamps.
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Cross-origin access and privacy
JavaScript access to cross-origin metrics
The PerformanceServerTiming interface is restricted by same-origin policy by default. For a cross-origin resource, the response can use Timing-Allow-Origin to allow specified origins to read timing information; browser restrictions may still apply. DevTools visibility does not mean page JavaScript can read the same information. Some browsers also require a secure context for this JavaScript interface. Check target-browser behavior and MDN’s compatibility and access notes.
Limit what public responses reveal
Metric names, descriptions, and durations can disclose details about application structure or infrastructure. Choose what to expose, to whom, and under what conditions. For public responses, avoid sensitive backend names or topology; where more detail is needed, consider limiting it to authenticated users or internal diagnostic tools. The header is for selected processing measurements, not a private channel.
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Troubleshoot missing or confusing metrics
- No header appears: Confirm that the server or intermediary measures the intended work and actually adds
Server-Timingto that response. The browser cannot generate server metrics on its own. - DevTools shows metrics but JavaScript does not: Check whether the request is cross-origin and whether its response permits access with an appropriate
Timing-Allow-Originpolicy. Also check secure-context requirements and the target browser. - The observer seems to miss earlier entries: The example uses
buffered: trueso observers can receive buffered entries. Confirm that you observe bothnavigationandresourcetypes relevant to your question. - Values do not match the browser’s total request time: They measure different scopes. Review what each server metric covers instead of summing component durations or expecting synchronized clocks.
- A duration looks high but the page is still slow elsewhere: Treat the value as a lead about its measured component. Inspect the browser timing and other relevant request or rendering evidence before deciding what caused the user-visible delay.
- Metrics appear on a trailer but not in Fetch: MDN says DevTools can display a Server-Timing HTTP trailer, while Fetch cannot access HTTP trailers. Do not assume trailer values are available through the Performance API.
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Availability and specification status
MDN describes the Server-Timing header as widely available across browsers since March 2023, while noting that details such as trailer handling and secure-context requirements can vary. Verify behavior in the browsers you support. The W3C publication dated April 7, 2026 is a Working Draft, not a Recommendation, and may change.
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