Why response time of a page does not equal the sum of its requests
The response time for a page typically differs from the
sum of its requests. This does not mean that your data is incorrect.
The difference can be caused by concurrent requests, page connection
times, inter-request delays, and custom code within a page.
The most common reason for the sum of the individual request times
within a page to exceed the total page response time is that requests
are often sent concurrently (in parallel) to a server. Thus some of
the individual request response times overlap so the sum of the request
response times would exceed the page response time.
Additionally, the page response time can exceed the sum of the
individual request response times within the page for the following
reasons:
The individual request response times do not include time to establish
connections but the page response time does include the connection
request time.
Inter-request delays are not reflected in the individual request
response time but are reflected in the page response time.
Custom code placed within a page is executed serially (after
waiting for all previous individual requests to complete) and thus
contributes to the page response time. It does not affect individual
request response times. However, we recommend that you place custom
code outside of a page, where it will not affect page response time.
For more information, see Reducing the performance impact of custom code.