start() {
    if (!PERF._started) {
      PERF._started = true;

      // register trackers after the current call stack completes
      _.defer(() => {
        PERF._start_tracking_draw_scene();
        PERF._start_tracking_loads();
        PERF._start_tracking_network();
      });
    }
  }
Starts profiling performance by tracking all predefined methods.