Why is macro so effing slow???

Here is the macro I wrote:

Sub RemoveNotAddRows()
    Dim bDelRow As Boolean
    Dim c, r, lastRow As Long
    Dim checkVals(30) As Integer
    Dim begSumCol, endSumCol As Integer
    Dim strTmp As String
    Dim wb As Workbook
    Dim ws As Worksheet

    With Application
        .ScreenUpdating = False
        .Calculation = xlManual
    End With

    Set wb = ActiveWorkbook
    Set ws = wb.Sheets("Sheet1")
    
    begSumCol = -99
    endSumCol = -99
    
    For c = 1 To 30
        strTmp = Left(ws.Cells(1, c), 4)
        If strTmp <> "Cell" Then
            If begSumCol = -99 Then
                begSumCol = c
            ElseIf Len(strTmp) = 0 Then
                endSumCol = c - 1
                Exit For
            End If
        End If
    Next c

    For c = begSumCol To endSumCol
        checkVals(c) = CInt(ws.Cells(1, c))
    Next c
    
    With ws
        lastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
    End With
    
    For r = lastRow To 2 Step -1
        bDelRow = False
        For c = begSumCol To endSumCol
            If CInt(ws.Cells(r, c)) <> checkVals(c) Then
                bDelRow = True
                Exit For
            End If
        Next c
        If bDelRow Then
            Rows(r).EntireRow.Delete
        End If
    Next r

    With Application
        .ScreenUpdating = True
        .Calculation = xlAutomatic
    End With
End Sub

Nothing special or fancy. Here's the issue: When I tried running this on a file with 43,352 rows and 7 columns of sums to match, I had to kill Excel after 10+ minutes because there were no results. However, when I manually checked the sums in two columns, and reduced the number of rows to 2170, the macro finished in 10 seconds. Simple match suggest that the original file should have finished in about three and one half minutes, but that didn't happen. Why?

update: I just ran this macro again on a different file with 'only' 12,046 rows. Based on the above math, this should have finished in under one minute, but it dd not finish until almost five minutes had elapsed.