DataGridView Cell Formatting Issue

[VisualBrian]

I'm using the following code to number the DGV rows in the Row HeaderCell...

    Private Sub DataGridViewTable_CellFormatting(sender As Object, e As DataGridViewCellFormattingEventArgs) Handles DataGridViewTable.CellFormatting
        DataGridViewTable.Rows(e.RowIndex).HeaderCell.Value = CStr(e.RowIndex + 1)
    End Sub

and it works, but the row header blinks like it's constantly cycling through the numbers and the app is sluggish.

What am I doing wrong?

EDIT...

Changed up the code a bit, removed it from CellFormatting and call it whenever the rowcount changes. All is well.

For rowNum As Integer = 0 To .Rows.Count - 1
    DataGridViewTable.Rows(rowNum).HeaderCell.Value = (rowNum + 1).ToString
Next

Still not sure why so if anyone knows please enlighten me.