Revert "PagePerformance: Wait for onload event to finish"

This reverts commit ffc4d85.

js/piwik.js

@@ -7353,10 +7353,9 @@ if (typeof window.Matomo !== 'object') {
              * Mark performance metrics as available, once onload event has finished
              */
             trackCallbackOnLoad(function(){
-                // Onload event is fired. Wait 15 seconds to assume it has also finished. */
                 setTimeout(function(){
                     performanceAvailable = true;
-                }, 15000);
+                }, 0);
             });
 
             Matomo.trigger('TrackerSetup', [this]);