It's quite often that you try to evaluate the performance bottle necks of a given Application Under Test (AUT) with the help of various performance metrics. You might capture these metrics with the help of performance testing tools like IBM Rational Performance Tester or LoadRunner or a wide variety of other tools.
The complexity arises when you see several complex requests in transactions and sometimes its confusing when the metric shows no values being captured. This puts you in a tricky situation and makes it difficult to come to a definitive conclusion. There might be response times captured against pages, Page Elements, and Transactions as a whole and they might be crucial during the computation of performance metrics. Let's try to analyze one small concept of how you can interpret Response time data against Elapsed time.
A page consists of several connection streams, executing page elements in parallel, where a page element is one part of the page such as an image or client-side script. Page element response time is the time from the first byte sent to last byte received. The response time for a page connection stream is the total of all of its page element response times plus any additional time establishing connections, if any. Page response time is the maximum response time of all page connection streams. Processing overhead time for data correlation, action scheduling delays under stress, Custom Code processing or HTTP processing are all excluded from page response time.
Each page element also has a delay associated with it. This delay is the time that was observed while recording the page. If one page element has a direct dependence on the response from another page element, RPT honors this dependence. The additional delay may or may not be significant but RPT includes it by default in an attempt to behave as much like the browser as possible. It may very well not be significant and you can therefore reduce it or eliminate it.
However, note that the 'Think Time' value set in the schedule is not included in the page response times displayed in the performance report. Additionally, it should be noted that there are potential "delays" in the script that could increase page response times. 'Think times' are associated with pages and are intended to represent human pauses during recording - these don"t impact page response times. For individual page elements (requests), there may also be delays which are intended to represent client (browser) delays (processing time, for example).
In more generic terms .......
Elapsed Time (Turn Around / Swing Time) = Request transmission time + Time of process in Server + Response Transmission Time
Response Time = Time to get the first response from server (Time to complete file downloading is elapsed time, whereas the time to start file downloading is called Response time).