Why is this locking up? Loop through all rows, perform function on duplicate, delete duplicate row

Question

The code works when I bite off a couple hundred rows at a time, but always hangs somewhere in the middle when I try to run it on 10,000.

What the code does:

Answer 1

Start with this and let us know how things are going afterwards:

Option Explicit

Sub combineDelete()

Const TEST_COLUMN As String = "A"
Dim i As Long
Dim iLastRow As Long
Dim s As Double, t As Double, u As Double
Dim v As Double, w As Double, y As Double

With Application
    .ScreenUpdating = False
    .Calculation = xlCalculationManual
    .EnableEvents = False
End With

With ActiveSheet
    iLastRow = .Cells(.Rows.Count, TEST_COLUMN).End(xlUp).Row
    For i = iLastRow To 2 Step -1
        If .Cells(i, 1).Value2 = .Cells(i - 1, 1).Value2 Then
            s = .Cells(i, 3).Value2
            t = .Cells(i - 1, 3).Value2
            .Cells(i - 1, 3).Value2 = s + t
            u = .Cells(i, 4).Value2
            v = .Cells(i - 1, 4).Value2
            .Cells(i - 1, 4).Value2 = u + v
            w = .Cells(i, 5).Value2
            y = .Cells(i - 1, 5).Value2
            .Cells(i - 1, 5).Value2 = w + y
            .Rows(i).EntireRow.Delete
        End If
    Next i
End With

With Application
    .EnableEvents = True
    .Calculation = xlCalculationAutomatic
    .ScreenUpdating = True
End With

End Sub

Notes:

1. Disable screenupdating, calculations and events
2. Use .Value2 instead of .Value
3. Explicit coding
4. Missing references to ActiveSheet added by adding dots .
5. Dim all variables to avoid variants