C# Mysql - Using of locks on database query for async await server

I have TcpListener class and I'm using async/await reading and writing.

For this server I have created single database instance where I have prepared all database queries.

But for more then one TcpClient I'm keep getting exception:

An exception of type MySql.Data.MySqlClient.MySqlException occurred in MySql.Data.dll but was not handled in user code

Additional information: There is already an open DataReader associated with this Connection which must be closed first.

If I understand it correctly there can't be more then one database query at time which is problem with more then one async client.

So I simply added locks in my queries like this and everything seems fine.

   // One MySqlConnection instance for whole program.

   lock (thisLock)
   {
    var cmd = connection.CreateCommand();

    cmd.CommandText = "SELECT Count(*) FROM logins WHERE username = @user AND password = @pass";
    cmd.Parameters.AddWithValue("@user", username);
    cmd.Parameters.AddWithValue("@pass", password);

    var count = int.Parse(cmd.ExecuteScalar().ToString());
    return count > 0;
}

I have also try the method with usings which create new connection for every query as mentioned from someone of SO community but this method is much more slower than locks:

    using (MySqlConnection connection = new MySqlConnection(connectionString))
    {
        connection.Open();   // This takes +- 35ms and makes worse performance than locks

        using (MySqlCommand cmd = connection.CreateCommand())
        {
            cmd.CommandText = "SELECT Count(*) FROM logins WHERE username = @user AND password = @pass";
            cmd.Parameters.AddWithValue("@user", username);
            cmd.Parameters.AddWithValue("@pass", password);

            int count = int.Parse(cmd.ExecuteScalar().ToString());
            return count > 0;
        }
    }

I used Stopwatch to benchmarks this methods and queries with one connection with locks are performed in +- 20ms which is +- only delay of network but with usings it is +- 55ms because of .Open() method which takes +- 35ms.

Why a lot of people use method with usings if there is much worse performance? Or am I doing something wrong?