webworker-threads slow transfer

index.js

var Worker = require('webworker-threads').Worker;

var workerCount = 4, finishedCount = 0, bufferSize = (1024 * 1024 * 4);
var workers = {};
var buffers = {};

function messageReceiver(event) {
  var data = event.data;
  var id = data.id;
  var buffer = data.buffer;
  console.log('Received', buffer[0], 'from', id);
  workers[id].terminate();
  if (++finishedCount === workerCount) {
    console.log('All workers finished.');
  }
}

for (var i = 0; i < workerCount; ++i) {
  console.log('Spawning worker', i);
  var worker = new Worker('worker.js');
  worker.onmessage = messageReceiver;
  workers[i] = worker;
}
console.log('All workers spawned.');

for (var i = 0; i < workerCount; ++i) {
  console.log('Allocating buffer', i);
  buffers[i] = new ArrayBuffer(bufferSize);
}
console.log('All buffers allocated.');

for (var i = 0; i < workerCount; ++i) {
  console.log('Launching worker', i);
  var w = workers[i];
  var a = buffers[i];
  var data = {
    id: i,
    buffer: new Uint32Array(a)
  };
  w.postMessage(data, [data.buffer]);
}
console.log('All workers launched.');

worker.js

function finishWorker(id, buffer) {
  console.log('Thread ' + id + ' finished filling buffer.');
  var data = {
    id: id,
    buffer: buffer
  };
  postMessage(data, [data.buffer]);
}

function fillBuffer(buffer) {
  for (var i = 0; i < buffer.byteLength; ++i) {
    if (i % (1024 * 1024) === 0) {
      console.log('Thread ' + id + ' filled ' + i + ' bytes');
    }
    buffer[i] = ((Math.random() * 255) | 0) % 255;
  }
}

this.onmessage = function (event) {
  var id = event.data.id, buffer = event.data.buffer;
  console.log('Thread ' + id + ' received buffer of ' + buffer.byteLength);
  fillBuffer(buffer);
  finishWorker(id, buffer);
};