Time to First Byte

Q: What is Time to First Byte?

Time to First Byte (TTFB) is a metric that reports the time between a request for a resource and the first byte arriving to the browser.

Q: How is Time to First Byte calculated?

Time to First Byte is the sum of several request and networking phases between the start of the request and the start of the response.

Q: What Time to First Byte measurement is fast?

Pages with an Time to First Byte of < 800 milliseconds can be considered as fast, where as durations over 800 milliseconds are considered slow.

Time to First Byte (TTFB) is a a metric that reports the time between a request for a resource and the first byte arriving to the browser.

How is Time to First Byte calculated?

Time to First Byte is a sum of several request and networking phrases:

redirect time
service worker startup time (if present)
DNS lookup
connection and TLS negotiation
the request (until the first byte has arrived)

Because a slow response time can have a cascading, adverse effect on overall web performance, we recommend optimising Time to First Byte as a priority.

What Time to First Byte measurement is fast?

Pages with Time to First Byte below or equal 800ms are considered as fast, and above 1800ms are considered slow.

Further resources

Time to First Byte: What it is and How to Make Improvements