Time to First Byte (TTFB)

 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.

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.

TTFB is not a Core Web 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.

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.

 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.