setting-up-computeds

controllers.application.js

import Ember from 'ember';

const items = Array.from(Array(250000), () => ({
  foo: 'hello',
  bar: 'goodbye'
}));

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  items,
  
  foos: Ember.computed('items.@each.foo', function() {
    return 'ok';
  })
});

helpers.log-on-render.js

import Ember from 'ember';

export function logOnRender([label]) {
  let now = performance.now();
  console.log(label, now);
  return `${label}: ${now}`;
}

export default Ember.Helper.helper(logOnRender);

templates.application.hbs

{{log-on-render 'before'}}
<br>
{{foos}}
<br>
{{log-on-render 'after'}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
  }
}