I doubt there will be a very appreciable performance difference, but you can avoid the name lookup on every row simply by lifting it out of the loop. This is probably the best you'll be able to achieve:
int idIdx = rdr.GetOrdinal("Id");
int createdIdx = rdr.GetOrdinal("Created");
while(rdr.Read())
{
var myObj = new myObj();
myObj.Id = rdr.GetFieldValue<int>(idIdx);
//more populating of myObj from rdr
myObj.Created = rdr.GetFieldValue<DateTime>(createdIdx);
}