VBA code optimization - excel

Question

I have a set of VBA codes which work really perfectly with around of 20 000 x 16 cells.
However, I need to use the codes with max 80 000 x 16 cells.
I have identified two types of codes which run really slow:
c = 2 'this is the first row where your data will output
d = 2 'this is the first row where you want to check for data
Application.ScreenUpdating = False
Do Until Range("A" & c) = "" 'This will loop until column U is empty, set the column to whatever you want
'but it cannot have blanks in it, or it will stop looping. Choose a column that is
'always going to have data in it.
ws1.Range("U" & d).FormulaR1C1 = "=RC[-20] & RIGHT(""0000"" & RC[-14], 6)"
c = c + 1 'Advances a and b when there is a matching case
d = d + 1
Loop
Application.ScreenUpdating = True
End Sub
Sub OpenValue()
Dim l As Integer
Dim k As Integer
Dim m As Integer
m = Sheets("Input").Range("AC:AC").End(xlDown).Row
For l = 2 To m
If Range("AC" & l) = "Delievered" Then
Range("AD" & l) = 0
ElseIf Range("AC" & l) = "Cancelled" Then
Range("AD" & l) = 0
Else
Range("AD" & l) = Val(Range("Z" & l)) * Val(Range("J" & l))
End If
Next
End Sub
What can I do to poptimize them ....

Answer 1

The link provided by #GSerg is an awesome way to cut the running time of your script down. I found myself using:
Application.ScreenUpdating set to False
Application.Calculation set to xlCalculationManual
Application.EnableEvents set to False
Application.DisplayAlerts set to False
so often that I combined them into a single public subroutine. #Garys-Student provided the inspiration:
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'INPUT : True or False (i.e. fast or slow)
'DESCRIPTION : this sub turns off screen updating and alarms then
' sets calculations to manual
'
Public Sub GoFast(OnOrOff As Boolean)
Dim CalcMode As XlCalculation
CalcMode = Application.Calculation
With Application
.ScreenUpdating = Not OnOrOff
.EnableEvents = Not OnOrOff
.DisplayAlerts = Not OnOrOff
If OnOrOff Then
.Calculation = xlCalculationManual
Else
.Calculation = CalcMode
End If
End With
End Sub
In practice, you can now add the one-liner:
Call GoFast(True)
at the beginning of your script as part of the setup, then add:
Call GoFast(False)
at the end of your script as part of the teardown. Modify as you see fit!

Answer 2

The Do Until can be replaced with a one liner:
ws1.Range("A2", ws1.Range("A2").End(xlDown)).Offset(0,20).FormulaR1C1 = _
"=RC[-20] & RIGHT(""0000"" & RC[-14], 6)"
Note that this will fail if A3 is empty. If you have headers in row 1 you can change the second A2 to A1.
For the other Sub I'm not sure if you are doing something special with Val but if not you could change it to something similar:
Sub OpenValue()
Dim r As Range
Set r = Sheets("Input").Range("AD2:AD" & Sheets("Input").Range("AC1").End(xlDown).Row)
With r
.FormulaR1C1 = "=IF(OR(RC[-1]=""Delivered"",RC[-1]=""Cancelled""),0,RC10*RC26"
'If you want these as values uncomment the following lines
'.Calculate
'.Copy
'.PasteSpecial xlPasteValues
End With
End Sub
Sprinkle Application stuff around if needed (Calculation, ScreenUpdating, DisplayAlerts, EnableEvents).
Why is this faster:
To put it simply, VBA and Excel have to open a 'channel' to communicate between each other and this costs some time. So looping through a Range and adding formulas one-by-one is much slower for large ranges than doing it all at once since you'll only open the 'channel' once.