This value represents the average amount of time it takes to load a page in your website and includes both the time it takes for your webserver to generate the page, and the time it takes for your visitor to download it. it is measured on the client browser using the Timing browser API
I would also like more detail on this. In particularly - how does Piwik handle async communications in this timing tracking? If we load up our Angular app, the GUI is immediately presented, while in the background some services are still loading data for use elsewhere in the app. Then there is a map - when the user is using the map he/she is generating requests to the servers to get more map data just by panning. Is each pan telling Piwik that something is happening, or slowing the time it seems that it takes for a page to finish loading… especially if a user starts panning before the page is finished.
Thanks for clarifying how Piwik handles async communication in tracking generation!