如何优化需要多次调用子例程的 VBA 模块？

Question

I am writing code for an Excel workbook provided by a client.

The purpose of this macro is to load the data for a customer into a sheet. When it does, the listener on one of the loaded cells is triggered, and it calls the appropriate subroutine associated with that customer's information.

The code is operating exactly as expected and completes in a few seconds. However, the client wishes us to perform this task on roughly 15,000 customers in sequence.

When we attempt to iterate the macro over roughly a hundred customers, the macro completes in a matter of a few minutes. However, when we increase the number of customers to around a thousand, the time to complete the run increases drastically.

I've been trying for days to optimize the subroutine itself, but the slowdown only arises when we are calling the subroutine upwards of a thousand times.

Why would repeated calls to the subroutine cause slowdown in the execution of the code, and is there a way that it can be mitigated?

Sub Looprun()

Dim n As Long
Dim lastRow As Long
Dim SecondLastrow As Long
Dim Start As Long
Dim Finish As Long

Application.Calculation = xlManual
Application.ScreenUpdating = False

'Set starting timestamp
Range("RunStart").Value = Now()

Start = Sheets("Reserve Summary").Range("L12").Value
Finish = Sheets("Reserve Summary").Range("L13").Value

lastRow = Cells(Sheets("Reserve Summary").Rows.Count, 2).End(xlUp).Row - 1

For n = Start To Finish
    
    Sheets("Calculation").Cells(3, 3).Value = n
' Cell C3 has a listener attached to it; see below. The customer's other information is loaded in via formulas that are keyed to cell C3.

    Sheets("Reserve Summary").Cells(n + 2, 3).Value = Sheets("Calculation").Range("C4").Value
    
    Sheets("Reserve Summary").Cells(n + 2, 5).Value = Sheets("Calculation").Range("Q25").Value
    
    Sheets("Reserve Summary").Cells(n + 2, 7).Value = Sheets("Calculation").Range("Q22").Value
    
Next

Range("RunEnd").Value = Now()

Application.Calculation = xlAutomatic
Application.Calculate
Application.ScreenUpdating = True

End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Dim company As String

    If Target.Address = "$C$3" Then
    
        Worksheets("Calculation").Calculate
        
        company = Sheets("Calculation").Cells(3, 11).Value
        
        If LCase(company) = "A" Then
            Call SubroutineA
            Exit Sub
        End If
        
        If LCase(company) = "B" Then
            Call SubroutineB
            Exit Sub
        End If
        
        Call DefaultSubroutine
    
    End If
    
End Sub