Performance and extensibility improvment for my Custom Authorization Module

I have an authorization requirement to have my security roles based on actions methods, which can not be achieved using the default asp.net mvc authorization. so i have created the following action filter, to implement my custom authorization requirments:-
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)] public class CheckUserPermissionsAttribute : ActionFilterAttribute { Repository repository = new Repository(); public string Model { get; set; } public string Action { get; set; } public override void OnActionExecuting(ActionExecutingContext filterContext) { // var user = User.Identity.Name; // or get from DB string ADusername = filterContext.HttpContext.User.Identity.Name.Substring(filterContext.HttpContext.User.Identity.Name.IndexOf("\\") + 1); if (!repository.can(ADusername,Model,Act ...