Optimize Slow VBA Code

Speed up code and stop screen flickering:

Sub NoScreenRePainting()
    Application.ScreenUpdating=False
　　'Your code here.
    Application.ScreenUpdating=True
End Sub

---

Preventing calculation while executing code:

Sub NoCalculations()
　　Application.Calculation = xlCalculationManual
　　'Your code here.
　　Application.Calculation = xlCalculationAutomatic
End Sub

---

Speeding up code if you have Worksheet or Workbook Events. Also stops endless loops in Events

Sub StopAllEvents()
　　Application.EnableEvents = False
　　'Your code here.
　　Application.EnableEvents = True
End Sub

---

Use the With Statement when working with Objects.

Sub WithARange()
　　With Range("A1")
　　　　.Value = 100
　　　　.Font.Bold = True
　　　　.Interior.ColorIndex = 6
　　　　.Copy Destination:=Range("B1")
　　End With
End Sub

---

Use VbNullString instead of = "" When needing to default a String variable back to it's default of "" use:

Sub EmptyText()
　　Dim strWords As String
　　strWords = "Cats"
　　MsgBox strWords
　　strWords = vbNullString
　　MsgBox strWords
End Sub

---

Inserting a Relative formula into a range of cells: Faster than AutoFill or Copy.

Sub NoAutoFillOrCopy()
　　Range("A1:A200").FormulaR1C1 = "=SUM(RC[1]:RC[5])"
End Sub

---

Tip: To get a formula, type it in any cell then select the cell, go Tools>Macro>Record new macro and record a macro pushing F2 then Enter.

Avoid the use of Copy and Paste whenever Possible:

Sub NoCopyAndPaste()
　　'Instead of:
　　Sheet1.Range("A1:A200").Copy
　　Sheet2.Range("B1").pasteSpecial
　　Application.CutCopyMode=False'Clear Clipboard
　　'Use:
　　'By-passes the Clipboard
　　Sheet1.Range("A1:A200").Copy Destination:=Sheet2.Range("B1")
　　'Or, if only values are needed:
　　Sheet2.Range("B1:B200").Value= Sheet1.Range("A1:A200").Value
　　'Or, if only formulae are needed:
　　Sheet2.Range("B1:B200").Formula = Sheet1.Range("A1:A200").Formula
　　'See also FormulaArray and FormulaR1C1 etc
　　'Instead of:
　　Sheet1.Range("A1:A200").Copy
　　Sheet1.Range("A1:A200").PasteSpecial xlPasteValues
　　Application.CutCopyMode=False'Clear Clipboard
　　'Use:
　　Sheet1.Range("A1:A200") = Sheet1.Range("A1:A200").Value
End Sub

---

Always declare your variables correctly!

To quickly view a variables definition:
Select the variable that you want the definition for.
Go to View>Definition (Shift+F2)

To return to your previous position:
Go to View>Last Postition (Ctrl+Shift+F2).

Release memory from Object variables:

Sub ReleaseObjectMemory()
　　'Could be any variable of the Object type
　　Dim wSheet as Worksheet
　　'Set Object variable
　　Set wSheet = Sheet1
　　'Your code here.
　　'Release memory
　　Set wSheet = Nothing
End Sub

---

Don't get caught in the Loop.
Follow this link to see why Loops should (and usually can) be avoided.

Avoid If, Else whenever possible

More often than not people would use an If, Else Statement to test whether a condition is TRUE or FALSE. There is however a slightly faster (and less typing) method. The first example shows the common method, while the second shows a faster method. Of course in such a small example the difference is not noticeable.

Sub TrueOrFalseSlower()
　　Dim bYesNo As Boolean
　　Dim i As Integer
　　If i = 5 Then
	bYesNo = True
　　Else
	bYesNo = False
　　End If
　　MsgBox bYesNo
End Sub

---

Here's a better way!

Sub TrueOrFalseFaster()
　　Dim bYesNo As Boolean
　　Dim i As Integer
　　bYesNo = (i = 5)
　　MsgBox bYesNo
End Sub

---

Another common need is to toggle a variable between True and False depending on a condition. For example:

Sub ToggleTrueOrFalseSlower()
　　Dim bYesNo As Boolean
　　If bYesNo = False Then
	bYesNo = True
　　Else
	bYesNo = False
　　End If
　　MsgBox bYesNo
End Sub

---

Here's a much better way

Sub ToggleTrueOrFalseFaster()
　　Dim bYesNo As Boolean
　　bYesNo = Not bYesNo
　　MsgBox bYesNo
End Sub

Source: http://www.ozgrid.com/VBA/SpeedingUpVBACode.htm