The average response time per URL can be lower than the average syn to syn-ack time because the two statistics are referenced at different points.
Reference points for the measurements. -TCP SYN/ACK - measured from SYN -->ACK received -TCP 1st byte - timed b/w SYN <--> 1st L7 packet -Response/url - GET ---> end of data (see below for more formation)
Minimum/Maximum/Average Time to TCP SYN/ACK (msec) The amount of time, in milliseconds, starting from the session issuing the SYN packet until the corresponding ACK is received. The Average Time To TCP SYN/ACK is the average of all TCP SYN/ACK times recorded over the sampling interval. The Minimum and Maximum Times To TCP SYN/ACK are the corresponding minimum and maximum times of those recorded among all previous sampling intervals.
Minimum/Maximum/Average Time to TCP First Byte (msec) The elapsed time, in milliseconds, between sending the SYN packet from the client and receiving the first Layer 7 packet from the device under test.
Minimum/Maximum/Average Response Time per URL (msec) The amount of time, in milliseconds, from the initial GET request until the requested URL is completely retrieved. For HTTP 1.0 with no Keep-Alive set, a URL is considered retrieved when a FIN is received from the server. For HTTP 1.1, the URL is completely retrieved when data packets totalling the Content-Length declared in the response header are received. For chunked transfers in HTTP 1.1, data is completely received when the server sends the end of chunk sequence: NOTES: - Even if you have only one URL in the Action list, it is possible to have a lower Response Time per Page than the Response Time per URL, especially when you have redirects and unsuccessful transactions. If you have only one URL and 100% successful transactions, the page response times and URL response times should match. - The Response Time per URL will not be affected by a 2-second ARP timeout, if present.