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To test a slow API in a frontend, measure both the browser’s request timings and the time it takes the user’s task to finish, then reproduce the conditions with Chrome DevTools throttling. These are different measurements: a request’s first-byte interval is not pure network latency, and a completed response does not necessarily mean the interface is usable.
Choose what you mean by “slow”
Before measuring, name the endpoint and the user flow, then decide which interval answers your question. At least three distinct timings matter:
- Request to first byte: how long from the browser starting the request until it receives the first response byte. This includes more than network transit; server processing and browser effects can contribute.
- Response download: how long the browser takes to receive the response body.
- User operation: how long from the user’s action until the intended result is rendered and usable.
Calling all three “latency” makes results hard to interpret. Browser resource timings describe browser-observed milestones; they are not a packet-level network trace. The Resource Timing API defines those milestones, while the requestStart reference describes the request-start timestamp.
Measure a browser request with Resource Timing
Use a PerformanceObserver to receive resource entries as they arrive. Set buffered: true to include entries recorded before the observer was created, and filter for the endpoint you care about rather than treating every page resource as the API under test.
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const observer = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
if (entry.initiatorType !== "fetch" && entry.initiatorType !== "xmlhttprequest") continue;
if (!entry.name.includes("/api/search")) continue;
console.log({
url: entry.name,
requestToFirstByteMs: entry.responseStart - entry.requestStart,
fetchDurationMs: entry.responseEnd - entry.fetchStart,
dnsMs: entry.domainLookupEnd - entry.domainLookupStart,
connectMs: entry.connectEnd - entry.connectStart,
redirectMs: entry.redirectEnd - entry.redirectStart,
});
}
});
observer.observe({ type: "resource", buffered: true });Interpret the intervals in context:
responseStart - requestStartapproximates request-to-first-byte duration. It is not automatically pure network latency.responseEnd - fetchStartcovers fetching the final resource. The entry’sdurationincludes redirects, so it can differ when redirects occur.domainLookupEnd - domainLookupStartis the browser-observed DNS interval;connectEnd - connectStartcan include transport connection and TLS-related setup;redirectEnd - redirectStartis the redirect interval.
Caching, service workers, and connection reuse can change what these fields represent. A zero or missing value is not necessarily a zero-millisecond stage: it may be unavailable, skipped, or hidden. For cross-origin resources, detailed timing fields generally require the response to expose them through Timing-Allow-Origin. Resource Timing’s buffer defaults to 250 entries; applications generating more entries should manage the buffer and respond to its full event. See the Resource Timing documentation and responseStart reference.
Measure when the user’s task is actually complete
A request finishing is not the same as results being ready to use. Add User Timing marks around a meaningful operation, such as submitting a search and displaying usable results.
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performance.mark("search-submit");
async function search(query) {
const response = await fetch(`/api/search?q=${encodeURIComponent(query)}`);
const results = await response.json();
renderResults(results);
// Call this after the results are rendered and usable.
performance.mark("search-usable");
performance.measure("search-to-usable", "search-submit", "search-usable");
}Choose the completion mark to match the user’s goal: for example, results displayed and interactive, rather than merely data received. The User Timing mark reference covers marks and measures.
If you can instrument the backend, ask it to emit relevant server-side phases in a Server-Timing response header. That can help distinguish server work from the rest of the browser-observed interval. Cross-origin timing visibility still depends on the applicable access policy. See the Server-Timing header reference.
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Reproduce slow network conditions in Chrome
Throttle the whole session
- Open Chrome DevTools and select the Network panel.
- Choose a network throttling preset, or configure a custom profile with explicit download speed, upload speed, and latency.
- For a first-visit test, disable the cache in DevTools. For a repeat-visit test, run separately with the intended warm-cache state.
- Repeat the same user action and record the selected profile, cache state, browser version, and device or CPU conditions.
DevTools throttling applies constraints during the session. It is useful for controlled reproduction, but a desktop’s CPU slowdown does not reproduce a mobile processor’s architecture. Validate on actual target devices and networks when those differences matter. Chrome’s Network panel documentation explains the available controls.
Throttle or block one request
When the rest of the page should stay under normal conditions, use Network request conditions to throttle or block a selected resource. This isolates a suspected API from unrelated assets. Chrome documents individual request throttling as available starting with Chrome 145; the feature and labels can vary by version. These rules are temporary to the open DevTools session. See Set conditions for network requests.
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Use Lighthouse for an automated page audit
Use Lighthouse when you want an automated page audit and prioritized performance guidance, rather than only an isolated endpoint measurement. Record the throttling mode: simulated throttling scales results, while DevTools throttling applies CPU and network constraints during the audit and takes longer. The modes are not interchangeable. The Lighthouse performance scoring documentation describes the distinction.
The Tool Desk
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Compare runs so the result means something
Keep the test conditions stable and write them down. At minimum, record:
- Browser and version, URL, endpoint, and the exact user action.
- Cache state (cold or warm), network profile, and CPU or device class.
- Backend and environment, including whether server timing was exposed.
- Whether the reported number is request-to-first-byte, full resource fetch, or user-operation completion.
Repeat observations and report a distribution, such as the median and tail percentiles, when you have enough samples to make that useful. The available browser guidance does not establish one universal sample count or pass/fail latency threshold. State service-level objectives separately from observed results, and do not present a target as though it were a measured value.
Quick Recap
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Quick wins for a faster PC:
Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →
