Find out how long an Ajax request took to complete

This is the proper way to do it.

$.ajax({
    url: 'http://google.com',
    method: 'GET',
    start_time: new Date().getTime(),
    complete: function(data) {
        alert('This request took '+(new Date().getTime() - this.start_time)+' ms');
    }
});

https://jsfiddle.net/0fh1cfnv/1/

@codemeit is right. His solution looks something like the following using jQuery for the ajax request. This returns the request time in milliseconds.

var start_time = new Date().getTime();

jQuery.get('your-url', data, function(data, status, xhr) {
        var request_time = new Date().getTime() - start_time;
});