If you need to measure how long something takes with Javascript you can use the Date() object to get a start timestamp and then compare it later on using a second Date() object. Although I’m sure there are other benchmarking plugins etc available this can be a quick and easy way to benchmark something in Javascript.
First get the start timestamp as follows. This is the current time in milliseconds.
var start = new Date().getTime();
Then do your processing and substract start from the current time in millseconds.
var elapsed = new Date().getTime() - start;
This will give you the time taken between the two points in time in milliseconds.