The api the grid data type uses collects the grid data the same way your parser does (though it doesn't have the check to make sure HttpContext.Current has a value). It is fairly fast the first time, but really slow when I run the built in grid control from inside my custom grid widget. Not sure why though, as no parameters are passed to this particular api. You can see the main code below (creating an accordion grid widget whose content can be controlled with a grid control; can be done with tools like leblender, but that window doesn't really give enough space for a full grid control; can expand the window, but then that leads to a lot of wasted space in most cases).

(function () {
    angular.module('umbraco')
        .directive('bmgAccordion', bmgAccordion);

    bmgAccordion.$inject = ['dataTypeResource', 'notificationsService'];
    function bmgAccordion(dataTypeResource, notificationsService) {
        var directive = {
            restrict: 'E',
            template: `
                <umb-control-group label="Theme" description="Theme to use when rendering the accordion.">
                    <api-picker api="/umbraco/backoffice/bmgaccordion/accordion/" ng-model="model.value"></api-picker>
                </umb-control-group>
                {{gridConfigs}}
                <ul class="accordion" ng-if="gridConfigs">
                    <li ng-repeat="item in model.items" ng-class="{active: activeIndex === $index}">
                        <div>
                            <label>Label</label>
                            <input type="text" ng-model="item.label" placeholder="Enter Label" />
                        </div>
                        <div class="grid-container">
                            <umb-editor model="gridConfigs[$index]"></umb-editor>
                            <umb-load-indicator ng-if="busy"></umb-load-indicator>
                        </div>
                    </li>
                </ul>
            `,
            link: link,
            scope: {
                model: '=ngModel',
                gridId: '='
            }
        };
        return directive;
        function link($scope, $element) {
            var cancelListener;
            if (!$scope.model) {
                $scope.model = {
                    theme: 'Default',
                    items: [{}]
                }
                $scope.activeIndex = 0;
            }
            if ($scope.gridId) {
                $scope.busy = true;
                dataTypeResource.getById($scope.gridId)
                    .then(function (data) {
                        $scope.gridConfigs = [];
                        for (var i = 0; i < $scope.model.items.length; i++) {
                            $scope.gridConfigs.push({
                                "config": {
                                    "items": data.preValues[0].value,
                                    "rte": data.preValues[1].value
                                },
                                "value": $scope.model.items[i],
                                "view": "grid",
                                "label": "Content",
                                "hideLabel": false,
                                "readonly": false,
                                "editor": "Umbraco.Grid"
                            });
                        }
                        if ($scope.activeIndex !== undefined) {
                            cancelListener = $scope.$watch('gridConfigs[' + $scope.activeIndex + '].value', function (newVal) {
                                console.log(newVal);
                                $scope.model.items[$scope.activeIndex] = newVal;
                            });
                        }
                    })
            }
            else {
                notificationsService.error('No grid id has been set.')
            }

            $scope.toggle = function (index) {
                if (cancelListener) {
                    cancelListener();
                    cancelListener = null;
                }
                if ($scope.activeIndex === index) {
                    $scope.activeIndex = null;
                }
                else {
                    $scope.activeIndex = index;
                    cancelListener = $scope.$watch('gridConfigs[' + index + '].value', function (newVal) {
                        $scope.model.items[$scope.activeIndex] = newVal;
                    });
                }
            };

            $scope.addItem = function () {
                $scope.model.items.push({});
                $scope.activeIndex = $scope.model.items.length - 1;
            };

            $scope.removeItem = function (index) {
                if ($scope.activeIndex === index) {
                    $scope.activeIndex = null;
                }
                $scope.model.items.splice(index, 1);
            };

            $scope.$on('$destroy', function () {
                if (cancelListener) {
                    cancelListener();
                }
            });
        }
    }
})();

It gets the grid configuration from a grid data type (stored in the gridConfigs array) and then passes that to the umb-editor directive. The grid control loads fine eventually, it's just really slow to display because the api call doesn't resolve quickly. The relevant bit, that triggers the api call can be seen here.