Updating angular $scope variable inside forEach loop

I have a quite extensive filter/faceted search application. Data are loaded via an API, and then displayed on the page using a ng-repeat. There's also a google map instance which has a marker associated with every item. So the application is quite demanding in terms of computation. I guess I might be overloading some clients with weaker machines in the end.

That's enough of background. I have got it working the way I want. Now I'm looking for performance gains and more effective ways to accomplish my goals.

Now I'm trying to figure out whether it is better to update a $scope variable (which is an array) inside a forEach loop, or update a private array and then update the $scope variable after the loop?

So this

$scope.myArrayWithAllTheItems = [];
$scope.myArrayWithItemKeysToBeDisplayed = [];

$http({method: 'GET', url: '/MYAPIURL'}).success(function(data, status, headers, config) {

if (angular.isArray(data)) {

    angular.forEach(data, function (item, key) {

        /* 
            some prework on item object here
        */

        //update scope variable with every forEach iteration
        $scope.myArrayWithAllTheItems.push({
            /*
            BIG OBJECT WITH MY DATA
                foo : bar,
                etc...
            */
        });

        //I store keys of the items that will be displayed on the page at any one moment
        //Changing filter controls will alter this array, not the big one containg all the data
        $scope.myArrayWithItemKeysToBeDisplayed.push(key);                    

    });


} else {
    //display error...
}

}).error(function(data, status, headers, config) {
    //display error...
});

versus this (I removed the comments and else branches, but otherwise the bit is identical except for 2 private arrays):

$scope.myArrayWithAllTheItems = [];
$scope.myArrayWithItemKeysToBeDisplayed = [];

$http({method: 'GET', url: '/MYAPIURL'}).success(function(data, status, headers, config) {

    if (angular.isArray(data)) {

        //private array variables to be update during the loop
        var _myArrayWithAllTheItems = [],
            _myArrayWithItemKeysToBeDisplayed =[];

        angular.forEach(data, function (item, key) {

            _myArrayWithAllTheItems.push({
            });

            _myArrayWithItemKeysToBeDisplayed.push(key);                    

        });

        //now update $scope variables only after the loop
        $scope.myArrayWithAllTheItems = _myArrayWithAllTheItems;
        $scope.myArrayWithItemKeysToBeDisplayed = _myArrayWithItemKeysToBeDisplayed;


    }
});