Increasing service timeout for the AuthenticationService in RIA Services

I have a SL4 LOB app, that uses RIA Services only for authentication (so far).

I use the default provider and service with minor additions (see below).

Some users with slow internet connections have complained that their logins开发者_JAVA技巧 are timing out, with error :

Load operation failed for query 'Login'.
[HttpRequestTimedOutWithDetail]

The login works fine on my connection.

So,

1. Is there a way to increase the timeout for the service operations of the AuthenticationService? I have seen many fixes for RIA services timeout, but all of them pertain to DomainDataContexts, not to the AuthenticationService itself.
2. Is it possible that those users who are getting the timeout, is due to them being behind a nasty firewall? Is it possible for them to access the site (port 80), download a Silverlight application, but that only the WCF service calls are blocked?

MyCustomMembershipProvider :

public class MyCustomMembershipProvider : SqlMembershipProvider
{

    public override MembershipUser CreateUser(string username, string password, 
    string email, string passwordQuestion, string passwordAnswer, bool isApproved, 
    object providerUserKey, out MembershipCreateStatus status)
  {
     var newUser = base.CreateUser(username, password, email, passwordQuestion, 
          passwordAnswer, false, providerUserKey, out status);
     if (newUser != null)
     {
        SendEmailToAdminForApproval(newUser);
     }

     return newUser;
  }

  private void SendEmailToAdminForApproval(MembershipUser user)
  {
       // Send a mail here
  }
}

And, AuthenticationService :

[EnableClientAccess]
public class AuthenticationService : AuthenticationBase<User>
{
  public List<User> GetUsers()
  {
     return System.Web.Security.Membership.GetAllUsers().Cast<User>().ToList();
  }
}