https://stackoverflow.com/questions/46829431/ember-preload-page-not-triggering-for-slow-model

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model(params){    
    return new Ember.RSVP.Promise((resolve)=>{
      Ember.run.later(()=> {
        resolve("Result application rendered");
      }, 3000);
    });
  }
});

templates.application-loading.hbs

Application Loading....

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
{{model}}
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}