This very simple example demonstrates why heavy CPU tasks is really bad idea for node.js (and browser javascript VM, of course)

node-cpu.js

function load () {
  for (var i=0; i<60000; ++i) {
    for (var j=60000; j>=0; --j) {
      a = i*j;
    }
  }
}

function cpuConsume(count, cpuLoadFunc) {
  var num = 1;
  return Promise
    .all(
      Array
        .apply(
          [],
          new Array(count)
        )
        .map(function () {
          return new Promise(function (resolve, reject) {
            var a;
            var big = big;
            console.log('Thread ' + num++ + ' started');
            cpuLoadFunc();
            resolve();
          });
        })
    );
}

Promise
  .resolve()
  .then(function () {
    console.time(oneThread);
    return cpuConsume(1, load);
  })
  .then(function () {
    console.timeEnd(oneThread);
    console.log('---------------------------------------');
    console.time(twoThreads);
    return cpuConsume(2, load);
  })
  .then(function () {
    console.timeEnd(twoThreads);
  })
  .catch(console.error);