Time to First Byte (TTFB)

Jeremy Wagner
Barry Pollard

Browser Support

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Source

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.

A diagram of network request timings. The phases from left to right are Redirect (which overlaps with Prompt for Unload), Cache, DNS, TCP, Request, Response, Processing, and Load. The associated timings are redirectStart and redirectEnd (which overlap with the Prompt for Unload's unloadEventStart and unloadEventEnd), fetchStart, domainLookupStart, domainLookupEnd, connectStart, secureConnectionStart, connectEnd, requestStart, responseStart, responseEnd, domInteractive, domContentLoadedEventStart, domContentLoadedEventEnd, domComplete, loadEventStart, and loadEventEnd.
A diagram of network request phases and their associated timings. TTFB measures the elapsed time between startTime and responseStart.

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.

Good TTFB values are 0.8 seconds or less, poor values are greater than 1.8 seconds, and anything in between needs improvement

How to measure TTFB

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

Field tools

Lab tools

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
});

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.

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.