Entity Framework async call blocking UI thread

I have been slowly trying to convert my code from using action delegates to the new Tasks in my WPF application. I like the fact that an await operation can run in the same method, greatly reducing the number of methods I need, enhancing readability, and reducing maintenance. That being said, I am having a problem with my code when calling EF6 async methods. They all seem to run synchronously and are blocking my UI thread. I use the following technologies/frameworks in my code:

• .NET Framework 4.5
• WPF
• MVVM Light 5.2
• Generic Unit Of Work/Repository Framework v3.3.5 https://genericunitofworkandrepositories.codeplex.com/
• Entity Framework 6.1.1
• SQL Server 2008 R2 Express

As an example, I have a LogInViewModel, with a command that executes after a button is clicked on my WPF application. Here is the command as initialized in the constructor:

LogInCommand = new RelayCommand(() => ExecuteLogInCommand());

Here is the command body:

private async void ExecuteLogInCommand()
{
     // Some code to validate user input

     var user = await _userService.LogIn(username, password);

     // Some code to confirm log in
}

The user service uses a generic repository object that is created using MVVM Light's SimpleIoC container. The LogIn method looks like this:

public async Task<User> LogIn(string username, string password)
{
    User user = await _repository.FindUser(username);

    if (user != null && user.IsActive)
    {
        // Some code to verify passwords
        return user;
    }

    return null;
}

And my repository code to log in:

public static async Task<User> FindUser(this IRepositoryAsync<User> repository, string username)
{
    return await repository.Queryable().Where(u => u.Username == username).SingleOrDefaultAsync();
}

The SingleOrDefaultAsync() call is Entity Framework's async call. This code is blocking my UI thread. I have read multiple articles from Stephen Cleary and others about async await and proper use. I have tried using ConfigureAwait(false) all the way down, with no luck. I have made my RelayCommand call use the async await keywords with no luck. I have analyzed the code and the line that takes the longest to return is the SingleOrDefaultAsync() line. Everything else happens almost instantaneously. I have the same problem when making other async calls to the DB in my code. The only thing that fixes it right away is the following:

User user = await Task.Run(() =>
{
    return _userService.LogIn(Username, p.Password);
});

But I understand this should not be necessary since the call I am making to the database is IO bound and not CPU bound. So, what is wrong with my application and why is it blocking my UI thread?