What is responseStart?
- responseStart is the time immediately after the browser receives the first byte of the response from the server, or from relevant application caches or from local resources.
HTTP Request / Response
When a resource is retrieved via the network (rather than the application cache) responseStart represents part of the HTTP request / response timeline.
The HTTP request / response timeline includes requestStart, responseStart and responseEnd.
Why measure responseStart?
Since responseStart 1 is the first moment when a browser has begun to receive a response, we can use it to measure some useful timings.
- It is the end time for time to first byte TTFB.
- It is the end time of request duration.
TTFB
Time to first byte is reported by most performance tools and can help a webmaster understand where problems exist with their slow pages.
responseStart is the end time for TTFB.
Combined with navigationStart TTFB can be calculated.
TTFB = responseStart - navigationStart
Request Duration
There is no official timestamp for the end of a request. Some tools may report how long a request took and it is often measured using responseStart as and end time.
Combined with requestStart, responseStart is used to measure the duration of a request.
Request duration = responseStart - requestStart
What happened just before responseStart?
Just prior to the responseStart, a request was made from the browser to the server (requestStart).
What happens immediately after responseStart?
Assuming all goes well, the next step is the browser getting the full response.
The next timestamp will be responseEnd.
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