removeChild性能低下

function testRemoveChild():void
        {
            var startTime:int = getTimer();
            var count:int = 9999;
            for (var i:int = 0; i < count; i++)
            {
                var sp:Sprite = new Sprite();
                addChild(sp);
            }

            trace("创建对象计算用时",getTimer()-startTime,"毫秒");

            startTime = getTimer();
            while(this.numChildren)
                this.removeChildAt(this.numChildren-1);;
//            while(this.numChildren)
//                this.removeChildAt(0);

            trace("移除对象计算用时",getTimer()-startTime,"毫秒");
        }
testRemoveChild();