A quick test to get some rough performance numbers for events. roughly these are the numbers: no events = 52ms, with events + self listener = 432ms, with events but no listeners = 1203ms, with events and checks for listeners = 1081ms. Speed difference from no events to with events are from x 8.3 to x 23.1!!!

events-tests.js

var util = require('util');
var events = require('events');

var testRuns = 10000000;

// A class without event emitter
var BlaClass0 = function() { };

BlaClass0.prototype.doCalc = function() {
  var bla = 3.14 * 1234567890;
  return bla;
};


// A class with an event emitter, that listens for it's own 'result' event
var BlaClass1 = function() {
  events.EventEmitter.call(this);
  this.on('result', function() {});
};
util.inherits(BlaClass1, events.EventEmitter);

BlaClass1.prototype.doCalc = function() {
  var bla = 3.14 * 1234567890;
  this.emit('result', bla);
};


// A similar class with an event emitter, that DOES NOT listen for it's own 'result' event
var BlaClass2 = function() {
  events.EventEmitter.call(this);
};
util.inherits(BlaClass2, events.EventEmitter);

BlaClass2.prototype.doCalc = function() {
  var bla = 3.14 * 1234567890;
  this.emit('result', bla);
};


// A class with an event emitter, that checks for listeners before emitting
var BlaClass3 = function() {
  events.EventEmitter.call(this);
};
util.inherits(BlaClass3, events.EventEmitter);

BlaClass3.prototype.doCalc = function() {
  var bla = 3.14 * 1234567890;
  if(this.listeners('result').length > 0) {
    this.emit('result', bla);
  }
};



var blaClass0 = new BlaClass0();
console.time('blaClass0.doCalc');
for(var i=0 ; i < testRuns ; i++) {
  blaClass0.doCalc();
}
console.timeEnd('blaClass0.doCalc');


var blaClass1 = new BlaClass1();
console.time('blaClass1.doCalc');
for(var i=0 ; i < testRuns ; i++) {
  blaClass1.doCalc();
}
console.timeEnd('blaClass1.doCalc');


var blaClass2 = new BlaClass2();
console.time('blaClass2.doCalc');
for(var i=0 ; i < testRuns ; i++) {
  blaClass2.doCalc();
}
console.timeEnd('blaClass2.doCalc');


var blaClass3 = new BlaClass3();
console.time('blaClass3.doCalc');
for(var i=0 ; i < testRuns ; i++) {
  blaClass3.doCalc();
}
console.timeEnd('blaClass3.doCalc');