Applications slow down when getting distinct of 50K+ records from datatable in C#

/// <summary>
   /// Generate Distinct Data Table from the Data Table Generated from server.
   /// </summary>
   Func<CancellationTokenSource, DataGridCollectionView, DataTable, DataTable> GetDistinctTable =
       (cancellationToken, DqmDataGridCollectionView, ResultDataTable) =>
       {

           var distinctCol = DqmDataGridCollectionView.SortDescriptions.ToList();
           if (distinctCol.Any())
           {

               var _distinctColumns = from n in distinctCol
                                      select new { col = n.PropertyName.Split('_')[1] }.col;

               var dtLinq20 = new DataTable();

               foreach (var item in _distinctColumns)
               {
                   var existingColumn = ResultDataTable.Columns[item];
                   var _dColumn = new DataColumn { ColumnName = existingColumn.ColumnName, DataType = existingColumn.DataType, DefaultValue = existingColumn.DefaultValue, AllowDBNull = existingColumn.AllowDBNull, Caption = existingColumn.Caption };
                   dtLinq20.Columns.Add(_dColumn);
               }

               DataTable DistinctDataTable = null;

               try
               {
                   DistinctDataTable = ResultDataTable.AsEnumerable().AsParallel().WithCancellation(cancellationToken.Token)
                    .Select(row =>
                    {
                        var newRow = dtLinq20.NewRow();
                        foreach (var item in _distinctColumns)
                        {
                            newRow[item] = row[item];
                        }

                        return newRow;
                    })
                    .Distinct(DataRowComparer.Default).CopyToDataTable();
               }
               catch (Exception)
               {
                   return null;
               }


               DistinctDataTable.TableName = "Distinct_TBL";

               return DistinctDataTable;
           }
           return null;
       };