A tip I picked up from Fluent 2014

v8-opt.js

// V8 initializes memory space for _x and _y
// in advance
function Klass1() {
  this._x = null;
  this._y = null;
}
var k1 = new Klass1()._x = 'hi';
 
function Klass2() {}
// dynamically defined property converts to a
// linked list in memory, making accessing
// _x slower
var k2 = new Klass2()._x = 'hi';