Adding rows to a datatable takes too long

Question

If reader.AsDataSet.Tables(0).Rows.Count > 0 Then

    Dim DistItemslist As New List(Of Eidos)
    Dim dgv As New DataGridView()
    Dim dic As New Dictionary(Of String, Integer)()
    Dim cellValue As String = Nothing
    Dim totalLU1 As Double = 0
    Dim totalreserved As Double = 0
    Dim distDT As New DataTable

    Dim distDR As DataRow = Nothing

    dgv.BindingContext = New BindingContext()
    dgv.DataSource = reader.AsDataSet.Tables(0)

    For Each col As DataGridViewColumn In dgv.Columns
        distDT.Columns.Add(dgv(col.Index, 0).Value)
        'DataGridView1.Columns.Add(dgv(col.Index, 0).Value, dgv(col.Index, 0).Value)
    Next

    dgv.Columns.Add("key", "key")
    dgv.Rows.RemoveAt(0)

    For i As Integer = 0 To dgv.Rows.Count - 1
        If IsDBNull(dgv(3, i).Value) Then
            dgv(3, i).Value = "0"
        Else
            dgv(3, i).Value = FormatDateTime(dgv(3, i).Value, DateFormat.ShortDate)

            If Split(dgv(3, i).Value, "/")(1) <> "02" Then
                dgv(3, i).Value = "30/" & Split(dgv(3, i).Value, "/")(1) & "/" & Split(dgv(3, i).Value, "/")(2)
            Else
                dgv(3, i).Value = "28/" & Split(dgv(3, i).Value, "/")(1) & "/" & Split(dgv(3, i).Value, "/")(2)
            End If
        End If
            dgv(10, i).Value = dgv(0, i).Value & "#" & dgv(1, i).Value & "#" & dgv(3, i).Value

    Next

    For i As Integer = 0 To dgv.Rows.Count - 1
        If Not dgv.Rows(i).IsNewRow Then
            cellValue = dgv(10, i).Value.ToString()

            If Not dic.ContainsKey(cellValue) Then
                dic.Add(cellValue, 1)
            Else
                dic(cellValue) += 1
            End If
        End If
    Next


    For Each pair In dic

        Dim query = (From row As DataGridViewRow In dgv.Rows
                     Where row.Cells(0).Value = Split(pair.Key, "#")(0) AndAlso row.Cells(1).Value = Split(pair.Key, "#")(1) AndAlso row.Cells(3).Value = Split(pair.Key, "#")(2)
                     Select row)

        For Each item In query
            totalLU1 = totalLU1 + CDbl(item.Cells(4).Value)
            totalreserved = totalreserved + CDbl(item.Cells(9).Value)
            'MsgBox(item.Cells(0).Value & " " & item.Cells(1).Value & " " & item.Cells(2).Value & " " & item.Cells(3).Value & " " & item.Cells(4).Value & " " & item.Cells(5).Value & " " & item.Cells(6).Value & " " & item.Cells(7).Value & " " & item.Cells(8).Value & " " & item.Cells(9).Value & " " & item.Cells(10).Value)
            'MsgBox(totalLU1)
            If item Is query.Last Then
                'DistItemslist.Add(New Eidos(item.Cells(0).Value, item.Cells(1).Value, item.Cells(2).Value, item.Cells(3).Value, totalLU1, item.Cells(5).Value, item.Cells(6).Value, item.Cells(7).Value, item.Cells(8).Value, totalreserved))
                'DataGridView1.Rows.Add(item.Cells(1).Value, item.Cells(2).Value, item.Cells(0).Value, item.Cells(3).Value, totalLU1, item.Cells(5).Value, item.Cells(6).Value, item.Cells(7).Value, item.Cells(8).Value, totalreserved)
                distDT.Rows.Add(item.Cells(0).Value, item.Cells(1).Value, item.Cells(2).Value, item.Cells(3).Value, totalLU1, item.Cells(5).Value, item.Cells(6).Value, item.Cells(7).Value, item.Cells(8).Value, totalreserved)
                totalreserved = 0
                totalLU1 = 0
            End If
        Next
    Next

    'DataGridView1.DataSource = DistItemslist

End If

Hello there, I would appreciate some help on this one. I'm trying to populate a datable or gridview direclty with the results from a linq query. Unfortunately it's taking too long to complete so I'm wondering if I'm iterating the dictionary in a wrong manner or is it something else which I'm entirely oblivious to...

Edit: The original datasource is about 6000 rows long and it's taken from an excel spreadsheet into a datagridview (dgv) instance. Notice how this assignment is completed within a fraction of a second.

When I'm iterating the dictionary though (For Each pair In dic) to get my custom rows in a datatable it takes a bit less that a minute to complete to produce around 2300 rows.

Edit: To add more context, data consists of items (rows) that have specific codes, dates, and locations among other. What I'm trying to attempt is to concatenate items that have these three elements in common into one thus reducing the size of the table, while adding together (sum) other elements of each item. (not just removing duplicates)

Updated code that uses dictionary as intended:

        Dim XLDic As New Dictionary(Of String, String)()
        Dim SumDT As New DataTable
        Dim value As String
        Dim key As String


        For i As Integer = 0 To XLDT.Columns.Count - 1
            XLDT.Columns(i).ColumnName = XLDT.Rows(0).Item(i).ToString
            SumDT.Columns.Add(XLDT.Rows(0).Item(i).ToString)
        Next

        XLDT.Rows().RemoveAt(0)

        For i As Integer = 0 To XLDT.Rows.Count - 1
            If IsDBNull(XLDT.Rows(i).Item(3)) Then
                XLDT.Rows(i).Item(3) = "-"
            Else
                XLDT.Rows(i).Item(3) = FormatDateTime(XLDT.Rows(i).Item(3), DateFormat.ShortDate)

                If Month(CDate(XLDT.Rows(i).Item(3))) <> 2 Then
                    XLDT.Rows(i).Item(3) = "30/" & Split(XLDT.Rows(i).Item(3), "/")(1) & "/" & Split(XLDT.Rows(i).Item(3), "/")(2)
                Else
                    XLDT.Rows(i).Item(3) = "28/" & Split(XLDT.Rows(i).Item(3), "/")(1) & "/" & Split(XLDT.Rows(i).Item(3), "/")(2)
                End If
            End If
        Next

        For i As Integer = 0 To XLDT.Rows.Count - 1
            key = XLDT.Rows(i).Item(0)
            value = XLDT.Rows(i).Item(4)
            If Not XLDic.ContainsKey(key) Then
                XLDic.Add(key, value)   
            Else
                XLDic(key) = XLDic(key) & "..."
            End If
        Next


        For Each pair In XLDic
            SumDT.Rows.Add(pair.Key, pair.Value)
        Next

        DataGridView1.DataSource = SumDT
    End If
End If```