Fake XMLHttpRequest producing slow progress events

fakeXMLhttpRequest.js

function fakeXMLHttpRequest() {
    var self = this;
    this.upload = {
        addEventListener: function(evt, cb) {
            self.progressListener = cb;
        }
    }
    this.open = function() {};
    this.overrideMimeType = function() {};
    this.loaded = 0;
    this.total = 100;
    this.send = function() {
        var interval = setInterval(function() {
            self.loaded += 10 + Math.random()*10;
            self.progressListener({
                loaded: Math.min(self.loaded, 100),
                total: self.total
            });
            if ( self.loaded >= 100 ) {
                self.loaded = 0;
                clearInterval(interval);
            }
        }, 250);
    }
}