Fetching data from database using dapper as a dynamic list.

I have tried mapping above dynamic data to custom list using my own custom data mapping method, but taking more time for 10.8 million records.

Model:

public class Data 
{
    public int ID { get; set; }
    public List<Attribute> Attributes { get; set; }
}

public class Attribute 
{
    public string ATTR { get; set; }
    public string ATTR_VALUE { get; set; }
}

Output:

List<Data>
        => ID = 1
        => Attributes
            [0] => ATTR = A
                => ATTR_VALUE = CAR
            [1] => ATTR = B
                => ATTR_VALUE = BUS
            [2] => ATTR = C
                => ATTR_VALUE = TRUCK
        => ID = 2
        => Attributes
            [0] => ATTR = A
                => ATTR_VALUE = CAR
            [1] => ATTR = B
                => ATTR_VALUE = BUS
            [2] => ATTR = C
                => ATTR_VALUE = TRUCK

can any one help me on this with LINQ and performance?