New Twiddle

calendar-cell.component\.js

import Component from '@glimmer/component';

export default class extends Component {
}

calendar-cell.template\.hbs

<div class="calendar-cell">
	{{log 'render cell'}}
	Calendar Cell: {{@value}}
</div>

calendar-grid.component\.js

import Component from '@glimmer/component';

export default class extends Component {
   get cells() {
     if (this.args.span == 'week') {
       return [1,2,3,4,5,6,7];
     }
     else {
       return [1];
     }
  } 
}

calendar-grid.template\.hbs

{{#each this.cells as |value|}}
	<CalendarCell @value={{value}}/>
{{/each}}

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = "why so many re-renders"
	get calendarSpan() {
		if (window.innerWidth < 700) {
      return 'day';
    }
    else {
      return 'week';
    }
  }
}

templates.application\.hbs

<CalendarGrid @span={{this.calendarSpan}}/>

twiddle\.json

{
  "version": "0.17.0",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": true,
    "_APPLICATION_TEMPLATE_WRAPPER": false,
    "_JQUERY_INTEGRATION": false
  },
  "options": {
    "use_pods": true,
    "enable-testing": false
  },
  "dependencies": {
    "ember": "3.17.0",
    "ember-template-compiler": "3.17.0",
    "ember-testing": "3.17.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}