Time to First Byte (TTFB)

{% BrowserCompat 'api.PerformanceResourceTiming.responseStart' %}

{% Aside %} Time to First Byte (TTFB) is a foundational metric for measuring connection setup time and web server responsiveness in both the lab and the field. It helps identify when a web server is too slow to respond to requests. In the case of navigation requests—that is, requests for an HTML document—it precedes every other meaningful loading performance metric. {% endAside %}

What is TTFB?

TTFB is a metric that measures the time between the request for a resource and when the first byte of a response begins to arrive.

TTFB is the sum of the following request phases:

• Redirect time
• Service worker startup time (if applicable)
• DNS lookup
• Connection and TLS negotiation
• Request, up until the point at which the first byte of the response has arrived

Reducing latency in connection setup time and on the backend will contribute to a lower TTFB.

What is a good TTFB score?

Because TTFB precedes user-centric metrics such as First Contentful Paint (FCP) and Largest Contentful Paint (LCP), it's recommended that your server responds to navigation requests quickly enough so that the 75th percentile of users experience an FCP within the "good" threshold. As a rough guide, most sites should strive to have Time To First Byte of 0.8 seconds or less.

{% Aside 'important' %}

TTFB is not a [Core Web Vitals](/vitals/) metric, so it's not absolutely necessary that sites meet the "good" TTFB threshold, provided that it doesn't impede their ability to score well on the metrics that matter.

Websites vary in how they deliver content. A low TTFB is crucial for getting markup out to the client as soon as possible. However, if a website delivers the initial markup quickly, but that markup then requires JavaScript to populate it with meaningful content—as is the the case with Single Page Applications (SPAs)—then achieving the lowest possible TTFB is especially important so that the client-rendering of markup can occur sooner.

Conversely, a server-rendered site that does not require as much client-side work could have a higher TTFB, but better FCP and LCP values than an entirely client-rendered experience. This is why the TTFB thresholds are a “rough guide”, and will need to be weighed against how your site delivers its core content.

{% endAside %}

How to measure TTFB

TTFB can be measured in the lab or in the field in the following ways.

Field tools

• Chrome User Experience Report
• web-vitals JavaScript library

Lab tools

• In the network panel of Chrome's DevTools
• WebPageTest

Measure TTFB in JavaScript

You can measure the TTFB of navigation requests in the browser with the Navigation Timing API. The following example shows how to create a PerformanceObserver that listens for a navigation entry and logs it to the console:

new PerformanceObserver((entryList) => {
  const [pageNav] = entryList.getEntriesByType('navigation');

  console.log(`TTFB: ${pageNav.responseStart}`);
}).observe({
  type: 'navigation',
  buffered: true
});

{% Aside 'caution' %} Not all browsers support PerformanceObserver or its buffered flag. To get the largest possible browser support, consider adopting the web-vitals package, which is discussed below. {% endAside %}

The web-vitals JavaScript library can also measure TTFB in the browser with less complexity:

import {onTTFB} from 'web-vitals';

// Measure and log TTFB as soon as it's available.
onTTFB(console.log);

Measuring resource requests

TTFB applies to all requests, not just navigation requests. In particular, resources hosted on cross-origin servers can introduce latency due to the need to set up connections to those servers. To measure TTFB for resources in the field, use the Resource Timing API within a PerformanceObserver:

new PerformanceObserver((entryList) => {
  const entries = entryList.getEntries();

  for (const entry of entries) {
    // Some resources may have a responseStart value of 0, due
    // to the resource being cached, or a cross-origin resource
    // being served without a Timing-Allow-Origin header set.
    if (entry.responseStart > 0) {
      console.log(`TTFB: ${entry.responseStart}`, entry.name);
    }
  }
}).observe({
  type: 'resource',
  buffered: true
});

The above code snippet is similar to the one used to measure the TTFB for a navigation request, except instead of querying for 'navigation' entries, you query for 'resource' entries instead. It also accounts for the fact that some resources loaded from the primary origin may return a value of 0, since the connection is already open, or a resource is instantaneously retrieved from a cache.

{% Aside 'gotchas' %} TTFB for cross-origin requests will not be measurable in the field if cross-origin servers fail to set a Timing-Allow-Origin header. {% endAside %}

How to improve TTFB

An in-depth guide on optimizing TTFB has been published to give you more guidance on improving your website's TTFB.