gistfile1.js

app.directive('faFastScroll', ['$parse', function ($parse) {
  var Interval = function(min, max) {
    this.min = min || 0;
    this.max = max || 0;
  };

  Interval.prototype.clip = function(min, max) {
    if(this.max <= min || this.min >= max) {
      this.min = this.max = 0;

      return;
    }

    this.min = Math.max(this.min, min);
    this.max = Math.min(this.max, max);
  };

  Interval.prototype.expand = function(i) {
    this.min -= i;
    this.max += i;
  };

  return {
    link: function (scope, element, attrs) {
      var cellHeight = parseInt(attrs.cellHeight, 10),
          getter = $parse(attrs.faFastScroll);

      function getVisibles(collection) {
        var offset = element.scrollTop(),
            range = element.height();

        // strictly visible bounds
        var visibles = new Interval(
            Math.floor(offset / cellHeight) - 1,
            Math.floor((offset + range - 1) / cellHeight)
        );

        // expand a bit to avoid flickers
        visibles.expand(15);
        visibles.clip(0, collection.length);

        return visibles;
      }

      function updatePartialView(needDigest) {
        var collection = getter(scope);

        if (!collection) {
          scope.partial = [];

          return;
        }

        var visibles = getVisibles(collection);

        scope.partial = collection.slice(visibles.min, visibles.max);

        scope.top = visibles.min * cellHeight;
        scope.bottom = (collection.length - visibles.max) * cellHeight;

        // updatePartialView will be called a lot when scrolling.
        // prevent $digest from propagating to individual items to save time.
        if (needDigest) {
          scope.$broadcast('suspend');
          scope.$digest();
          scope.$broadcast('resume');
        }
      }

      element.on('scroll', function () {
        updatePartialView(true);
      });

      scope.$watchCollection(attrs.faFastScroll, function () {
        // we're already in a $digest
        updatePartialView(false);
      });
    }
  };
}]);

app.directive('faSuspendable', function () {
  return {
    link: function (scope) {
      // FIXME: this might break is suspend/resume called out of order
      // or if watchers are added while suspended
      var watchers;

      scope.$on('suspend', function () {
        watchers = scope.$$watchers;
        scope.$$watchers = [];
      });

      scope.$on('resume', function () {
        scope.$$watchers = watchers;
        watchers = void 0;
      });
    }
  };
});

It's a good point, doesn't look like it would do any harm to me.

for history. link for post https://coderwall.com/p/d_aisq/speeding-up-angularjs-s-digest-loop