X-Timer
Timing information about the response.
Fastly writes this header into responses. It is proprietary to Fastly.
S{unixStartTimeSeconds},VS0,VE{durationMilliseconds}{unixStartTimeSeconds} refers to the number of seconds since the Unix epoch at which the request was first received by Fastly. It is given in seconds but to microsecond precision.
{durationMilliseconds} is an integer number of whole milliseconds elapsed since the start time of the request. In the event of a cache hit, this will most likely be zero, since the time required to look up and serve a cached object is usually under a millisecond.
'VS' and 'VE' refer to 'varnish start' and 'varnish end', but the VS number is always zero.