X-Timer
Timing information about the response.
Fastly writes this header into responses. It is proprietary to Fastly.
S{unixStartTimeSeconds},VS0,VE{durationMilliseconds}{unixStartTimeSeconds} is a Unix timestamp for when the request was first received by Fastly. It is given in seconds but to microsecond precision.
The value {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. For a cache MISS, the number represents the number of milliseconds it took to retrieve the data from your origin server and send the response back to the requester.