# Time to First Byte (TTFB)

Category: [Performance & Core Web Vitals](https://seo-geo-glossary.com/en/glossary/category/performance-core-web-vitals/) › [Core Web Vitals & metrics](https://seo-geo-glossary.com/en/glossary/category/performance-core-web-vitals/core-web-vitals-metrics/) · Also known as: TTFB, server response time

**Time to First Byte (TTFB)** is the time between the start of navigation to a page and the moment the first byte of the response arrives. web.dev describes it as covering redirects, service worker start-up if present, the DNS lookup, connection and TLS setup, and the request up to the first response byte. MDN gives the same core idea and the formula `responseStart` from the Navigation Timing API.

## What are the thresholds?

web.dev lists:

- **Good:** 0.8 seconds or less
- **Needs improvement:** 0.8 to 1.8 seconds
- **Poor:** more than 1.8 seconds

TTFB is not a Core Web Vital. It comes before First Contentful Paint (FCP) and Largest Contentful Paint (LCP), so a slow TTFB adds to both. For LCP, web.dev puts TTFB at about 40 percent of the typical total.

## How is it measured?

In the field with the Chrome User Experience Report or the `web-vitals` library, in the lab with Chrome DevTools or WebPageTest. Tools do not all measure the same span. According to DebugBear, the Lighthouse audit for server response time leaves out connection setup. In JavaScript:

```javascript
const ttfb = performance.getEntriesByType("navigation")[0].responseStart;
```

With HTTP `103 Early Hints`, MDN notes that the value may reflect the early response and not the final one.

## What causes a slow TTFB?

DebugBear groups the causes:

- **Server work:** heavy template rendering, many plugins.
- **Database:** slow or repeated queries, missing indexes.
- **Network:** distance between visitor and server.
- **Dependencies:** calls to third-party APIs, cold starts of serverless functions.

Typical fixes are caching HTML at a CDN, optimizing queries and reducing redirects.

## Example

A shop page shows a TTFB of 1.2 seconds in field data, which is in the "needs improvement" range. Most of that time is spent waiting for the server to build the page. Caching the rendered HTML at the CDN edge lowers the wait.

## At a glance

- **Measures:** Time from the start of navigation until the first byte of the response arrives
- **Unit:** Seconds
- **Assessment:** 0.8 s or less is good, above 1.8 s is poor
- **Data source:** Field: Chrome User Experience Report, web-vitals library; lab: DevTools, WebPageTest
- **Core Web Vital:** No, helps diagnose LCP
- **Docs:** [web.dev: Time to First Byte](https://web.dev/articles/ttfb)

Last checked: October 8, 2026

## Thresholds

- **TTFB** (Time to First Byte): good ≤ 0.8 s, needs improvement ≤ 1.8 s, poor > 1.8 s ([Source](https://web.dev/articles/ttfb))

Assessed at the 75th percentile of page loads, mobile and desktop separately.

## Web Vitals metrics compared

| Metric | Measures | Core Web Vital? | Field or lab | “Good” per web.dev |
|---|---|---|---|---|
| LCP (Largest Contentful Paint) | Render time of the largest image, text block or video visible in the viewport, from the start of navigation | yes (loading) | field and lab | 2.5 s or less |
| INP (Interaction to Next Paint) | Latency of interactions across the whole visit until the next paint; one value that all (or nearly all) interactions fall below | yes (interactivity), since 2024-03-12 | field; in the lab only with simulated interactions, Lighthouse does not measure INP | 200 ms or less |
| CLS (Cumulative Layout Shift) | Largest burst of unexpected layout shifts during the entire lifecycle of the page | yes (visual stability) | field and lab | 0.1 or less |
| TTFB (Time to First Byte) | Time from the start of navigation until the first byte of the response arrives | no (helps diagnose LCP) | field and lab | 0.8 s or less |
| FCP (First Contentful Paint) | Time from the start of navigation until any part of the page's content is rendered | no (helps diagnose LCP) | field and lab | 1.8 s or less |
| TBT (Total Blocking Time) | Total time after FCP during which the main thread was blocked long enough to prevent input responsiveness | no (lab proxy for INP) | lab; web.dev does not recommend field measurement | less than 200 ms on average mobile hardware |
| Speed Index | How quickly content is visually displayed during page load (from a video of the load) | no | lab (Lighthouse) | Threshold according to Lighthouse: mobile up to 3.4 s “fast” (web.dev gives no value) |
| FID (First Input Delay), historical | Time from the first interaction until the browser can begin processing event handlers | until 2024-03-12, replaced by INP | field only | 100 ms or less |

Sources: [web.dev: Web Vitals](https://web.dev/articles/vitals); [web.dev: Largest Contentful Paint (LCP)](https://web.dev/articles/lcp); [web.dev: Interaction to Next Paint (INP)](https://web.dev/articles/inp); [web.dev: Cumulative Layout Shift (CLS)](https://web.dev/articles/cls); [web.dev: Time to First Byte (TTFB)](https://web.dev/articles/ttfb); [web.dev: First Contentful Paint (FCP)](https://web.dev/articles/fcp); [web.dev: Total Blocking Time (TBT)](https://web.dev/articles/tbt); [web.dev: First Input Delay (FID)](https://web.dev/articles/fid); [web.dev Blog: Interaction to Next Paint is officially a Core Web Vital (2024-03-12)](https://web.dev/blog/inp-cwv-launch); [Chrome for Developers: Lighthouse – Speed Index](https://developer.chrome.com/docs/lighthouse/performance/speed-index). Last checked: October 8, 2026

## Related terms

- [Server response time](https://seo-geo-glossary.com/en/glossary/server-response-time/index.md)
- [First Contentful Paint (FCP)](https://seo-geo-glossary.com/en/glossary/first-contentful-paint/index.md)
- [Largest Contentful Paint (LCP)](https://seo-geo-glossary.com/en/glossary/largest-contentful-paint/index.md)
- [CDN (content delivery network)](https://seo-geo-glossary.com/en/glossary/cdn/index.md)
- [Browser caching](https://seo-geo-glossary.com/en/glossary/browser-caching/index.md)
- [Core Web Vitals](https://seo-geo-glossary.com/en/glossary/core-web-vitals/index.md)

## Sources

- [Time to First Byte (TTFB) – web.dev](https://web.dev/articles/ttfb)
- [Time to First Byte – MDN Web Docs](https://developer.mozilla.org/en-US/docs/Glossary/Time_to_first_byte)
- [Time to First Byte – DebugBear](https://www.debugbear.com/docs/metrics/time-to-first-byte)
- [Optimize Time to First Byte – web.dev](https://web.dev/articles/optimize-ttfb)
- [Web Vitals – web.dev](https://web.dev/articles/vitals)

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Last updated: October 8, 2026
URL: https://seo-geo-glossary.com/en/glossary/time-to-first-byte/