Testing the load on the browser using many ScriptProcessor nodes. Chrome Canary v26.0.1367.0 suggests that around 15 ScriptProcessor nodes performing simple functions is enough to noticeably disrupt the audio output.

ScriptProcessorLoad.js

var context = new webkitAudioContext()
  , source = context.createOscillator()
  , through = context.createScriptProcessor(1024, 1, 1)
  , count = 0;

// The most basic of ScriptProcessing functions, copying the input buffer
// directly into the output buffer.
through.onaudioprocess = function (e) {
  var input = e.inputBuffer.getChannelData(0)
    , output = e.outputBuffer.getChannelData(0);
  
  for (var i = 0; i < output.length; i++) {
    output[i] = input[i];
  }
};

// Make the connections
source.connect(through);
through.connect(context.destination);

// Start it up
source.start(0);

// Every second, route an additional ScriptProcessor into the node graph
// to test the event loop. These additional ScriptProcessors do not route
// any output into the output buffers so that you can hear exactly when the
// load becomes overbearing.
setInterval(function () {
  console.log(++count);
  
  var next = context.createScriptProcessor(1024, 1, 1)
    , fn = function (e) {
        var input = e.inputBuffer.getChannelData(0);
        for (var i = 0; i < input.length; i++) {
          input[i] += 0.01;
        }
      };
      
  window.fns = window.fns || [];
  window.fns.push(fn);
  
  next.onaudioprocess = fn;
  
  source.connect(next);
  next.connect(context.destination);
  
}, 1000);

It's not working. I can't detect any data passing. I grab data from the audio element though.