It's not possible to directly measure time on last page visited.
To solve this a heartbeat signal was introduced which pings to piwik as long as site is visited " I'm still there!" see #8225
This produces additional server-load.
The more exact the time on page has to be measured (high resolution), the higher needed ping frequency and wit this the higher server-load.
Since not during complete visit a a high resolution is always necessary, one can work with growing intervals of pings see #9423
But of course even this produces permanent server-load.

An other way to help to measure time on last page is to use available "leaving signals" to send a signal "I'm leaving" and stop time recording when receiving this signal.
( instead of "I'm still there" heartbeat procedure)

possible leaving signal maybe:

• use outgoing link (in same tab)
• move fast mouse to top left (same as used to show advertisements an some sites)
• possibly some other...