Why asynchronous code is so much slower than synchronous code

Question

I create a test table in SQL Server like this:

CREATE TABLE Person
(
    PersonID INT NOT NULL IDENTITY,
    PersonName NCHAR(500) NOT NULL,
    PersonFamily NCHAR(500) NOT NULL,
 
    CONSTRAINT PK_Person PRIMARY KEY(PersonID)
)

Then I create two methods to test async.

The first method uses an async approach to read all person, and the second one uses a sync approach.

Async method:

private async Task<List<Person>> ReadPersonsAsync()
{
    List<Person> result = new List<Person>();

    using (SqlConnection sqlConnection = new SqlConnection(ConnectionString))
    {
        using (SqlCommand sqlCommand = new SqlCommand())
        {
            sqlCommand.CommandText = "SELECT * FROM Person";
            sqlCommand.CommandType = System.Data.CommandType.Text;
            sqlCommand.Connection = sqlConnection;

            await sqlConnection.OpenAsync();

            using (var reader = await sqlCommand.ExecuteReaderAsync())
            {
                while (await reader.ReadAsync())
                {
                    result.Add(new Person
                    {
                        ID = int.Parse(reader[0].ToString()),
                        Name = reader[1].ToString(),
                        Family = reader[2].ToString()
                    });
                }
            }
        }
    }

    return result;
}

Sync method:

private List<Person> ReadPersons()
{
    List<Person> result = new List<Person>();

    using (SqlConnection sqlConnection = new SqlConnection(ConnectionString))
    {
        using (SqlCommand sqlCommand = new SqlCommand())
        {
            sqlCommand.CommandText = "SELECT * FROM Person";
            sqlCommand.CommandType = System.Data.CommandType.Text;
            sqlCommand.Connection = sqlConnection;

            sqlConnection.Open();

            using (var reader = sqlCommand.ExecuteReader())
            {
                while (reader.Read())
                {
                    result.Add(new Person
                    {
                        ID = int.Parse(reader[0].ToString()),
                        Name = reader[1].ToString(),
                        Family = reader[2].ToString()
                    });
                }
            }
        }
    }

    return result;
}

When I use the async approach it takes 10 seconds, but the sync approach is run in 2 seconds.

Which part of my code is wrong? Thanks.

Answer 1

SqlClient implementation, today, is sync first. The async implementation of it has a lot of workarounds to make it work, some of them even work synchronously under the hood, so, the problem is the SqlClient design and the weight of its legacy code.

Recently, they started a project, SqlClientX to address this problem heavily. You can take a look here and follow the evolution of it. https://github.com/dotnet/SqlClient/discussions/2603#discussioncomment-9868402