Using HTML5's Navigation Timing API to measure Page Load speed

To measure the time taken for a page to load one can use the Date object in the head section and calculate the time taken for the page to load in window.onload function. However, it calculates the time taken for the page to load after the first bit of JavaScript in the head is executed. This doesn't indicate the time taken by the web server to serve the requested page. The Navigation Timing API introduced in HTML5 provides data that can be used to measure the performance of a website. The API provides a PerformanceTiming interface which contains several attributes that can be used to get end-to-end latency data.

Measuring Page load performance using the Date object:

As mentioned above, when you try to calculate the time taken for the page to load using the Date object, it doesn't indicate the time taken by the web server to server the page:

 var startTime = new Date().getTime();  
 function onLoadEventHandler() {  
      var latency = new Date().getTime() - startTime;  
      console.log('Latency = ' + latency + 'ms');  
 }  

This would output:

Latency = 2ms 

Measuring Page load performance using the performance.timing object:

The PerformanceTiming interface includes several attributes and one of them is the navigationStart attribute. This attribute returns the time when the browser attempts to fetch a new resource from the server. If there is a document from which the user is navigating to a new document (by clicking a link), then it returns time when the browser finishes unloading the previous document:

 var startTime = new Date().getTime();  
 function onLoadEventHandler() {  
      var latency = startTime - performance.timing.navigationStart;  
      console.log('Latency = ' + latency + 'ms');  
 }  

This would output:

Latency = 104ms

As you can see from the two examples, there is a difference of more than 100ms when using the performance.timing object.

PerformanceTiming interface:

The PerformanceTiming interface includes several attributes:

You can refer to more detailed explanation of attributes here.

Browser support:

Navigation Timing API is available on Chrome, Firefox 7 and on IE 9.