excel – Combine two rows into one based on matching ref very slow

I think what’s taking it slow is the multiple copy and paste wherein you can just do it in one go.
Also, If you are checking Column 4 only, then just loop there.
Another important thing is you cannot delete the row after you copy it.
The rows will move and then you will not get your expected results.
Try to get those rows first and delete in one go after you finished the iteration.
Try something a bit cleaner and direct:

Edit1: After reviewing your code, it seems you are trying to combine duplicates in the same row.

Sub CombineRows()
    Dim RowNum As Long, LastRow As Long
    Dim c As Range, rngtodelete As Range
    Application.ScreenUpdating = False
    With Sheets("Sheet1")
        RowNum = 2
        LastRow = .Range("A" & Rows.Count).End(xlUp).Row
        For Each c In .Range("D2:D" & LastRow) 'Loop in D column only
            If c.Value2 = c.Offset(1, 0).Value2 Then
                'Cut and paste in one go
                c.Offset(1, -3).Resize(, 10).Cut .Range("K" & RowNum)
                'Mark the rows to delete
                If rngtodelete Is Nothing Then
                    Set rngtodelete = c.Offset(1, 0).EntireRow
                Else
                    Set rngtodelete = Union(rngtodelete, c.Offset(1, 0).EntireRow)
                End If
            End If
            RowNum = RowNum + 1
        Next
        If Not rngtodelete Is Nothing Then rngtodelete.Delete xlUp 'Delete in one go
    End With
    Application.ScreenUpdating = True
End Sub

You can also learn a lot if you read this POST.
I don’t really know if this is what you’re trying to achieve.
I based it solely on the code you posted. This took less than a second in my machine. HTH.

You should try this:

Sub CombineRows()
    'define variables
    Dim RowNum As Long, LastRow As Long
    Application.ScreenUpdating = False
    'start below titles and make full selection of data
    RowNum = 2
    LastRow = Range("A" & Rows.Count).End(xlUp).Row
    'Range("A2", Cells(LastRow, 10)).Select
    'For loop for all rows in selection with cells
    'For Each Row In Selection
    '    With Cells
        'if order number matches
    With Worksheets("ABC") ' Whatever is the Tab name
        For RowNum = 2 To LastRow
            If .Cells(RowNum, 4) = .Cells(RowNum + 1, 4) Then
                'move attribute 2 up next to attribute 1 and delete empty line
                .Range(.Cells(RowNum + 1, 1), .Cells(RowNum + 1, 10)).Copy _
                        Destination:=.Range(.Cells(RowNum, 11), .Cells(RowNum, 20))
                'Cells(RowNum + 1, 1).Copy Destination:=Cells(RowNum, 11)
                'Cells(RowNum + 1, 2).Copy destination:=Cells(RowNum, 12)
                'Cells(RowNum + 1, 3).Copy destination:=Cells(RowNum, 13)
                'Cells(RowNum + 1, 4).Copy destination:=Cells(RowNum, 14)
                'Cells(RowNum + 1, 5).Copy destination:=Cells(RowNum, 15)
                'Cells(RowNum + 1, 6).Copy destination:=Cells(RowNum, 16)
                'Cells(RowNum + 1, 7).Copy destination:=Cells(RowNum, 17)
                'Cells(RowNum + 1, 8).Copy destination:=Cells(RowNum, 18)
                'Cells(RowNum + 1, 9).Copy destination:=Cells(RowNum, 19)
                'Cells(RowNum + 1, 10).Copy destination:=Cells(RowNum, 20)
                Rows(RowNum + 1).EntireRow.Delete
            End If
        Next
        'End With
    End With
        'increase rownum for next test
        RowNum = RowNum + 1
    'Next Row
    'turn on screen updating
    Application.ScreenUpdating = True
End Sub