Node.js Performance using Cluster

I have been trying to figure this out for a while. I wrote a very simple http server in node to benchmark the effect of using cluster. Here is my code:

var cluster = require('cluster');
var http = require('http');
var numCPUs = 0; //require('os').cpus().length;

if(process.argv.length >= 3)
{
  numCPUs = process.argv[2];
}

if (cluster.isMaster && numCPUs > 0) {
  console.log("launching " + numCPUs + " procs");
  // Fork workers.
  for (var i = 0; i < numCPUs; i++) {
    console.log("launching proc #" + i);
    cluster.fork();
  }

  cluster.on('death', function(worker) {
    console.log('worker ' + worker.pid + ' died');
  });
} else {
  // Worker processes have a http server.
  http.Server(function(req, res) {
    res.writeHead(200);
    res.end("hello world\n");
  }).listen(3000);
}

The problem is that I am not seeing any performance gain at all. 1 process has better performance most of the time. And, If I add more work, like retrieving data from redis or mongo then increasing the processes helps, but only modestly (around 15%). I've tried this on an i7 MBPr (quad-core with HT), and an i5 (quad-core) Win7 systems both with the same results.

Can someone please explain whats wrong with this code? Or, why am I not seeing an advantage/benefit in using cluster?