WCF Authentication and Authorization in 1 database call

Question

I have a WCF service set up, which currently implements an IAuthorizationPolicy and UserNamePasswordValidator, which, by themselves, are currently working to a basic degree.

It has been implemented into my web.config like so:

<serviceCredentials>
            <userNameAuthentication userNamePasswordValidationMode="Custom"
                                    customUserNamePasswordValidatorType="SecureService.CustomUsernamePasswordValidator, SecureService"/>            
</serviceCredentials>
<serviceAuthorization principalPermissionMode="Custom">
     <authorizationPolicies>
              <add policyType="SecureService.MyAuthorization, SecureService"/>
     </authorizationPolicies>
</serviceAuthorization>

What this is currently doing is validating the credentials in the UsernamePasswordValidator, and then retrieving the roles by checking the database for said user in a different call.

The issue I am having is that my 'database call' is a call to another application, where the calls themselves take about a second each. Therefore, I'd like to both authenticate and authorize with a single call.

The only possibility I have thought of is to put the logic entirely in the IsInRole function, which is currently set up as so:

public bool IsInRole(string role)
{
    PopulateUserRoles();
    return _roles.Contains(role);
}

My question is - is there a standard way to achieving this? And if not, is this a safe way to do both at once?