I'm not sure if it will improve performance, but you can use Starling's touch events to get the stage mouse coords.
Try something like:
private var stageCoords:Point = new Point();
stage.addEventListener(TouchEvent.TOUCH, onTouch);
private function onTouch(e:TouchEvent):void {
var touches:Vector.<Touch> = e.getTouches(stage);
if(touches.length > 0){
stageCoords = touches[0].getLocation(stage);
}
}