Effect of Screen Updating

Turning off screen updating will only make a difference to execution time if the code interacts with Excel in a way that causes changes to the screen content. The greater the amount of screen changes the bigger the impact will be. The other posted answers aptly demonstrate this.

Other application settings that can make a difference to execution time are Calculation and Event handling. Use this code template as a starting point (the error handler ensures that these properties are turned back on at the end of the sub, even if it errors)

Sub YourSub()    On Error GoTo EH    Application.ScreenUpdating = False    Application.Calculation = xlCalculationManual    Application.EnableEvents = False    ' Code hereCleanUp:    On Error Resume Next    Application.ScreenUpdating = True    Application.Calculation = xlCalculationAutomatic    Application.EnableEvents = TrueExit SubEH:    ' Do error handling    Resume CleanUpEnd Sub

Other techniques exist that can provide even greater improvement in execution speed.

The most useful include

1. Avoid Select, Activate and ActiveCell/Sheet/Workbook as much as possible. Instead declare and assign variables and reference those.
2. When referencing large ranges, copy the Range data to a variant array for processing and copy the result back to the range after.
3. Use Range.SpecialCells, Range.Find and Range.AutoFilter to limit the number of cells referenced.

There are plenty of examples of these techniques on SO.

If you want to see a fairly drastic example of why ScreenUpdating is important, run the following code. It takes roughly 45 times longer in Excel 2011 for me to run this swap without ScreenUpdating = false! This is a huge difference in time.

Sub testScreenUpdating()    Dim i As Integer    Dim numbSwitches As Integer    Dim results As String    'swap between sheets this number of times    numbSwitches = 1000    'keep track of time    Dim startTime As Double    startTime = Time    'swap between sheets 1/2 (need both sheets or this will crash)    For i = 1 To numbSwitches        Sheets(1 + (i Mod 2)).Select    Next i    'get results    results = "Screen Updating not disabled: " & Format(Time - startTime, "hh:mm:ss") & " seconds"    startTime = Time    'scenario 2 - screenupdating disabled    Application.ScreenUpdating = False    'swap between sheets 1/2 (need both sheets or this will crash)    For i = 1 To numbSwitches        Sheets(1 + (i Mod 2)).Select    Next i    Application.ScreenUpdating = True    'get results for part two    results = results & vbCrLf & "Screen Updating IS disabled: " & Format(Time - startTime, "hh:mm:ss") & " seconds"    'show results    MsgBox resultsEnd Sub

Also, while we're on the topic of ways to increase efficiency, another key point is that Select, Selection, and Activate are rarely (if ever) necessary. When you record macros it will always use these but there are very few situations when you need to actually use them in code. Likewise, anything with Active in title (such as ActiveCell) normally is an indication you will have slower code because you presumably are selecting cells.

You can almost always refer to cells/worksheets specifically and avoid select. For example:

msgbox (Worksheets(1).Range("A1").value) 

will work regardless of whether you are currently on the first worksheet. A common new VBA mistake is to do something more like:

Worksheets(1).Selectmsgbox (Range("A1").value)

which is an unneeded step.

This adds significant time to code runtimes.

Firstly I've been using the script written by Richie (UK) Post #7 Here

It simply iterates through a loop changing the value of i in one cell. I have changed it slightly so it loops 10,000 times and I execute it 10 times for sample size.

What is the effect of screen updating on the speed of my codes execution?

These are the lengths of execution when Screen Updating was disabled and enabled:

Disabled    Enabled0.61909653  2.1050669130.619555829 2.1068653630.620805767 2.1068663150.625528325 2.1024033150.625319976 2.09911790.621287448 2.1051031420.621540236 2.1013926650.624537531 2.1068667160.620401789 2.109004449

As you can see it takes almost 3.5 times as long to execute the code when Screen Updating is not disabled.

Both of these codes were exceuted using the Run button in the VB editor, as opposed to 'watching' the spreadsheet.

2 simple lines at the start and end of your code:

Application.ScreenUpdating = FalseApplication.ScreenUpdating = True

But, they can have a large effect on the efficiency of your execution!

Note: Obviously the advantages of Screen Updating will be well known to many here but this may be of benefit to beginners and I find it interesting to view the numbers!