One of the simplest ways to have a rough idea of how long a piece of code is taking to execute is by using the console methods time, timerLog, and timeEnd.

Example:

console.time("answer time");
// where `answer time` is a label to identify this timer
alert("Click to continue");
console.timeLog("answer time");
alert("Do a bunch of other stuff...");
console.timeEnd("answer time");

Most of the time you just want to time a whole event and don't care about intermediate steps, though. Then, you just need console.time, console.timeEnd.

That's pretty much it.