how to avoid nested for looping for improved performance?

public List LoadDataGridViewToTestUpdate()
{
DataEntities ob = new DataEntities();
var result = (from c in ob.SampleJSONTable
select c);
return result.ToList();
}

private DataTable LoadDataToTable()
{
Repository obj = new Repository();
List tableData = obj.LoadDataGridViewToTestUpdate();


var dataTable = new DataTable();

dataTable.Columns.Add(new DataColumn("Col1", typeof(int)));
dataTable.Columns.Add(new DataColumn("Col2", typeof(string)));
dataTable.Columns.Add(new DataColumn("Col3", typeof(string)));
//dataTable.Columns.Add(new DataColumn("Col4", typeof(string)));



foreach (var item in tableData.ToList())
{
var dataRow = dataTable.NewRow();

dataRow["Col1"] = item.Col1;
dataRow["Col2"] = item.Col2;
dataRow["Col3"] = item.Col3;

IEnumerable deserializedProduct = Newtonsoft.Json.JsonConvert.DeserializeObject>(item.Col4);

foreach (var jsonItem in deserializedProduct)
{

if (!dataTable.Columns.Contains(jsonItem.Name.ToString()))
{
dataTable.Columns.Add(new DataColumn(jsonItem.Name.ToString(), typeof(int)));
}
dataRow[jsonItem.Name.ToString()] = jsonItem.Vol;
}

dataTable.Rows.Add(dataRow);
}

return dataTable;
}
}