diff -r 000000000000 -r 6474c204b198 toolkit/devtools/server/actors/eventlooplag.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/devtools/server/actors/eventlooplag.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,90 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * The eventLoopLag actor emits "event-loop-lag" events when the event
+ * loop gets unresponsive. The event comes with a "time" property (the
+ * duration of the lag in milliseconds).
+ */
+
+const {Ci, Cu} = require("chrome");
+const Services = require("Services");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+const protocol = require("devtools/server/protocol");
+const {method, Arg, RetVal} = protocol;
+const events = require("sdk/event/core");
+
+exports.register = function(handle) {
+  handle.addGlobalActor(EventLoopLagActor, "eventLoopLagActor");
+  handle.addTabActor(EventLoopLagActor, "eventLoopLagActor");
+};
+
+exports.unregister = function(handle) {
+  handle.removeGlobalActor(EventLoopLagActor);
+  handle.removeTabActor(EventLoopLagActor);
+};
+
+let EventLoopLagActor = protocol.ActorClass({
+
+  typeName: "eventLoopLag",
+
+  _observerAdded: false,
+
+  events: {
+    "event-loop-lag" : {
+      type: "event-loop-lag",
+      time: Arg(0, "number") // duration of the lag in milliseconds.
+    }
+  },
+
+  /**
+   * Start tracking the event loop lags.
+   */
+  start: method(function() {
+    if (!this._observerAdded) {
+      Services.obs.addObserver(this, 'event-loop-lag', false);
+      this._observerAdded = true;
+    }
+    return Services.appShell.startEventLoopLagTracking();
+  }, {
+    request: {},
+    response: {success: RetVal("number")}
+  }),
+
+  /**
+   * Stop tracking the event loop lags.
+   */
+  stop: method(function() {
+    if (this._observerAdded) {
+      Services.obs.removeObserver(this, 'event-loop-lag');
+      this._observerAdded = false;
+    }
+    Services.appShell.stopEventLoopLagTracking();
+  }, {request: {},response: {}}),
+
+  destroy: function() {
+    this.stop();
+    protocol.Actor.prototype.destroy.call(this);
+  },
+
+  // nsIObserver
+
+  observe: function (subject, topic, data) {
+    if (topic == "event-loop-lag") {
+      // Forward event loop lag event
+      events.emit(this, "event-loop-lag", data);
+    }
+  },
+
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+});
+
+exports.EventLoopLagFront = protocol.FrontClass(EventLoopLagActor, {
+  initialize: function(client, form) {
+    protocol.Front.prototype.initialize.call(this, client);
+    this.actorID = form.eventLoopLagActor;
+    client.addActorPool(this);
+    this.manage(this);
+  },
+});