It does not matter how large your for loop is, AngularJS doesn't render anything to HTML unless the loop has finished. But it should render to HTML as $scope changes (as happens in two-way data binding). A pen is created for this.
Have I misunderstood anything?
You can force re-render manually with $timeout, see codepen: https://codepen.io/anon/pen/eWNmap - it's important that separated function was created due to asynchronous $timeout i always has 20 value because of JS closure: https://stackoverflow.com/a/750506/3368498 But this behavior is OK, angular re-render when it starts digest cycle - and if it will start with any change, it will be crazy slow. All in all, you can think about it as you wrote, when 2-way binding change it is re-rendered - cases when this behavior matches are very rare.
