Time to First Byte (TTFB) is the time elapsed between a browser sending an HTTP request and receiving the first byte of the server's response. It reflects server processing time, network latency, and any overhead from CDNs or proxies. A high TTFB delays everything that follows — the browser cannot begin rendering until it receives data. Google recommends keeping TTFB below 800 ms; values under 200 ms are considered fast. TTFB directly affects [Largest Contentful Paint (LCP)](/glossary/lcp) and therefore [Core Web Vitals](/glossary/core-web-vitals) scores.