Response Time Checker / Glossary

Response time glossary

Every number the Response Time Checker reports: what it is, how we measure it, what good looks like and how to improve it.

• TTFB (time to first byte)The time from the start of the request until the first byte of the response arrives.
• DNS lookupThe time to turn the hostname into an IP address.
• TCP connectThe time for the TCP handshake with the server: one round trip.
• TLS handshakeThe time to agree on encryption after the TCP connection is open.
• Server wait (server time)The time between sending the request and receiving the first byte, minus one network round trip: an estimate of how long the server spent building the response.
• Download timeThe time from the first byte to the last byte of the response.
• Redirect timeThe time spent on responses that only redirect (301, 302, 307, 308) before the final page.
• CDN cache hitWhen a CDN has the page in its cache (a HIT), it answers from an edge near the visitor without asking your server.
• Server-TimingA response header in which the server reports its own timings, for example Server-Timing: db;dur=53, render;dur=12.
• CompressionContent-Encoding shrinks text responses (HTML, CSS, JavaScript, JSON) before they travel, often to a quarter of their size.
• HTTP/2 and HTTP/3HTTP/2 sends many requests over one connection at once; HTTP/3 does the same over QUIC, which avoids head-of-line blocking and connects faster on lossy networks.
• Cold and warm requestsA cold request starts from nothing: DNS lookup, TCP connection and TLS handshake before the request itself.