Slowness on my code while looping through rows - excel

Question

Set WB = Thisworkbook
Set wk = WB.Sheets("Final")
Set ws = WB.Sheets("OIT-Temp")
irow = wk.Range("B65000").End(xlUp).Row 'Find last row
For i = 2 To irow Step 1
If not wk.Cells(i, 2).FormulaR1C1 = "#N/A" Then
wk.cells(i, 2).copy ws.Cells(i, 2)
wk.cells(i,3).Value = 128
Else
wk.Cells(i, 3).Value = 198
End if
Next i
I'm here facing too much of lag while running this code and unable to understand what's wrong with my code

Answer 1

Please, try the next adapted code. It uses arrays, mostly working in memory and drops the processed arrays at once. That's why it should be very fast:
Sub speedUpCode()
Dim WB As Workbook, wk As Worksheet, ws As Worksheet, iRow As Long, arr, arrCopy, i As Long
Set WB = ThisWorkbook
Set wk = WB.Sheets("Final")
Set ws = WB.Sheets("OIT-Temp")
iRow = wk.Range("B" & wk.rows.count).End(xlUp).row 'Find last row
arr = wk.Range("B2:C" & iRow).Value2 'place the range in an array for faster iteration
arrCopy = ws.Range("B2:B" & iRow).Value2 'place the range in an array for faster iteration
For i = 1 To UBound(arr)
If Not CStr(arr(i, 1)) = "Error 2042" Then ''#N/A
arrCopy(i, 1) = arr(i, 1)
arr(i, 2) = 128
Else
arr(i, 2) = 198
End If
Next i
'drop the processed array values:
wk.Range("B2").Resize(UBound(arr), 2).Value2 = arr
ws.Range("B2").Resize(UBound(arrCopy), 1).Value2 = arrCopy
End Sub

Answer 2

Some general points:
Indent your code.
Give your variables meaningful names.
Normal to write positive code block in if/else first.
I'm not sure how you're doing an xlUp on an entire row.
For this specific problem, reading and writing cells is computationally expensive. You can remove a lot of this by reading and writing data in bulk and working with your data in arrays in memory.
Try this and see if it speeds things up.
Sub FastReadWrite()
' Set the range references.
With ThisWorkbook
With .Sheets("Final")
Dim maxRow As Long
maxRow = .Range("B65000").End(xlUp).Row
Dim finalData As Range
Set finalData = .Range("B2:C" & maxRow)
End With
Dim oitTemData As Range
Set oitTemData = .Sheets("OIT-Temp") _
.Range("B2:C" & maxRow)
End With
' Read the data from the ranges into memory.
Dim fArray() As Variant: fArray = finalData.Formula2R1C1
Dim tArray() As Variant: tArray = oitTemData.Formula2R1C1
' Main logic loop.
Dim i As Long
For i = 1 To UBound(fArray, 1)
If fArray(i, 1) = "#N/A" Then
fArray(i, 2) = 198
Else
tArray(i, 1) = fArray(i, 1)
fArray(i, 2) = 128
End If
Next i
' Write the data to the ranges.
finalData.Formula2R1C1 = fArray
oitTemData.Formula2R1C1 = tArray
End Sub