Avg page load time metric is zero

[gerasyana]

I use GA to track page views and and site activity. I see Avg. Page Load Time metric is zero for most of pages. Can you tell me why the metric is calculated for specific pages only? Do I need to add other params when setting up account?

Code :

AnalyticsProvider.setAccount({
    name: 'ofSiteflowTracker',
    tracker: 'UA-XXXXXXXX-1',
    trackEvent: true,
    fields: {
        siteSpeedSampleRate: 100
    }
});
AnalyticsProvider.setPageEvent('$stateChangeSuccess');

"angulartics": "1.0.3", "angulartics-google-analytics": "0.1.4",