Elapsed Time is less than Total Time

Whenever I look at the Page Summary for a page I see that the Network->Elapsed Time is less than Timings->Network->Total

Why is this? Shouldn’t the sum of the network times of each request on the page be equal to the Elapsed Time?

The difference in the values is due to the fact that the browser often runs network requests concurrently rather than strictly one after the other.

The Elapsed Time is the time interval from the start of the first request to the end of the last request on a page (see help on the network tab). However, during this time several requests may overlap meaning that you can’t simply add their durations together to find the elapsed time for the whole page. The time chart below has a highlighted group of requests that are running concurrently.