If some javascript code takes too much time to execute, you probably want to see what time your code or a part of the code takes to run. I found an idea in Dave Cranes Ajax in Action. The book is a little bit old and the code provided below has been changed and simplified:

var stopwatch = {};
stopwatch.watches = \[\];
stopwatch.getWatch = function(id, startNow) {
	var watch = stopwatch.watches\[id\];
	if (!watch) {
		watch = new stopwatch.StopWatch(id);
	}
	if (startNow) {
		watch.start();
	}
	return watch;
};
stopwatch.StopWatch = function (id) {
	this.id = id;
	stopwatch.watches\[id\] = this;
	this.events = \[\];
	this.total = 0;
	this.count = 0;
};
stopwatch.StopWatch.prototype.start = function() {
	this.current = new stopwatch.TimedEvent();
};
stopwatch.StopWatch.prototype.stop = function() {
	if (this.current) {
		this.current.stop();
		this.events.push(this.current);
		this.count++;
		this.total += this.current.duration;
		this.current = null;
	}
};
stopwatch.TimedEvent = function () {
	this.start = new Date();
	this.duration = 0;
};
stopwatch.TimedEvent.prototype.stop = function() {
	var stop = new Date();
	this.duration = stop - this.start;
};