Sql – Why is Linq To Sql databinding to gridview much slower than pass-through SQL

I have compared two queries which fetch some fairly large data from a database table. For query one I used Linq To Sql and for the other I use passthrough SQL via ADO.NET.

I know that Linq To Sql has to do a lot of work behind the scenes, but what is it actually doing? The two queries fetches the same amount of data but the Linq To Sql query is more than 5 seconds slower and uses 150mb more RAM!

Here is my test code:

Using Linq To Sql :

public void MakeList()
    {
        int start = Environment.TickCount;
        var document = from d in _dm.tDokuments select d;

        List<tDokument> documentList = document.ToList();
        int end = Environment.TickCount;

        GridView1.DataSource = documentList;
        GridView1.DataBind();

        Label1.Text = (end - start).ToString();
    }

Passthrough SQL + ADO.NET:

public void MakeList()
    {

        int start = Environment.TickCount;
        SqlCommand sqlCommand = new SqlCommand("SELECT * FROM tDokument", _connection);
        SqlDataAdapter da = new SqlDataAdapter(sqlCommand);

        DataSet ds = new DataSet();
        da.Fill(ds);
        int end = Environment.TickCount;

        GridView1.DataSource = ds;
        GridView1.DataBind();

        Label1.Text = (end - start).ToString();
    }