My VBA code is getting slower every iteration - excel

Question

I'm using the code to add a column, bucket the value on left (via a vlookup) and then auto fill the whole column. The problem I'm having is every time I use the macro, the code is taking slightly longer and longer. Would appreciate any help :)
Here is the code:
Sub insert_col()
'
' insert_col Macro
'
' Keyboard Shortcut: Ctrl+w
'
Dim x As Variant
Dim a As Long
Dim b As Long
Dim y As Variant
Dim t As Single
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
t = Timer
ActiveSheet.Columns(ActiveCell.Column).EntireColumn.Select
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
x = ActiveCell.Column
Cells(22, x).Select
ActiveCell.FormulaR1C1 = "=VLOOKUP(RC[-1],R13C1:R193C2,1)"
Cells(22, x).Select
a = ActiveCell.Column
x = ActiveCell.Row
y = ActiveCell.End(xlDown).Row
Selection.AutoFill Destination:=Range(Cells(x, a), Cells(36600, a)), Type:=xlFillDefault
ActiveCell.Offset(0, 2).Select
MsgBox Timer - t
Application.ScreenUpdating = True
Application.Calculation = xlCalculationAutomatic
End Sub
Thanks!

Answer 1

To be clearer on the comment above, your code adds additional formulae each time it is run which increases the calculation time.
You can greater simply the code along the lines below:
Sub insert_col()
' Keyboard Shortcut: Ctrl+w
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
t = Timer
ActiveCell.EntireColumn.Insert
Range(Cells(22, ActiveCell.Column), Cells(36600, ActiveCell.Column)).FormulaR1C1 = "=VLOOKUP(RC[-1],R13C1:R193C2,1)"
ActiveCell.Offset(0, 2).Select
MsgBox Timer - t
Application.ScreenUpdating = True
Application.Calculation = xlCalculationAutomatic
End Sub