Today’s author, Chad Rothschiller, a Program Manager on the Excel team, is back with a follow up from hisprevious post on VBA and Excel performance.
I want to start off this post by thanking everyone who sent in their examples in response to my January request. It is incredibly helpful to be able to look at what you all are doing with Excel! Not only did I see a huge variety in how Excel is being used, you also pointed out various tips and tricks for writing fast VBA code in Excel.
In this post I'm going to share with you the most important performance tips I know about. There are tons of sites, pages, and people who are experts as well on this subject, have performed their own tests, and shared their results and ideas. If you think I missed an important concept for how to optimize Excel VBA performance, or if you've got a valuable comment or link to share, please feel free to post here so everyone can benefit. Thanks!

Turn Off Everything But the Essentials While Your Code is Running
This optimization explicitly turns off Excel functionality you don't need to happen (over and over and over) while your code runs. Note that in the code sample below we grab the current state of these properties, turn them off, and then restore them at the end of code execution.
One reason this helps is that if you're updating (via VBA) several different ranges with new values, or copy / pasting from several ranges to create a consolidated table of data, you likely do not want to have Excel taking time and resources to recalculate formulas, display paste progress, or even redraw the grid, especially after every single operation (even more so if your code uses loops). Just one recalculation and one redraw at the end of your code execution is enough to get the workbook current with all your changes.
Here's some sample code that shows how and what to shut off while your code runs. Doing this should help improve the performance of your code:
'Get current state of various Excel settings; put this at the beginning of your code
screenUpdateState = Application.ScreenUpdating
statusBarState = Application.DisplayStatusBar
calcState = Application.Calculation
eventsState = Application.EnableEvents
displayPageBreakState = ActiveSheet.DisplayPageBreaks 'note this is a sheet-level setting
'turn off some Excel functionality so your code runs faster
Application.ScreenUpdating = False
Application.DisplayStatusBar = False
Application.Calculation = xlCalculationManual
Application.EnableEvents = False
ActiveSheet.DisplayPageBreaks = False 'note this is a sheet-level setting
'>>your code goes here<<
'after your code runs, restore state; put this at the end of your code
Application.ScreenUpdating = screenUpdateState
Application.DisplayStatusBar = statusBarState
Application.Calculation = calcState
Application.EnableEvents = eventsState
ActiveSheet.DisplayPageBreaks = displayPageBreaksState 'note this is a sheet-level setting
Here's a quick description for each of these settings:
Application.ScreenUpdating: This setting tells Excel to not redraw the screen while False. The benefit here is that you probably don't need Excel using up resources trying to draw the screen since it's changing faster than the user can perceive. Since it requires lots of resources to draw the screen so frequently, just turn off drawing the screen until the end of your code execution. Be sure to turn it back on right before your code ends.
Application.DisplayStatusBar: This setting tells Excel to stop showing status while False. For example, if you use VBA to copy/paste a range, while the paste is completing Excel will show the progress of that operation on the status bar. Turning off screen updating is separate from turning off the status bar display so that you can disable screen updating but still provide feedback to the user, if desired. Again, turn it back on right before your code ends execution.
Application.Calculation: This setting allows you to programmatically set Excel's calculation mode. "Manual" (xlCalculationManual) mode means Excel waits for the user (or your code) to explicitly initiate calculation. "Automatic" is the default and means that Excel decides when to recalculate the workbook (e.g. when you enter a new formula on the sheet). Since recalculating your workbook can be time and resource intensive, you might not want Excel triggering a recalc every time you change a cell value. Turn off calculation while your code executes, then set the mode back. Note: setting the mode back to “Automatic” (xlCalculationAutomatic) will trigger a recalc.
Application.EnableEvents: This setting tells Excel to not fire events while False. While looking into Excel VBA performance issues I learned that some desktop search tools implement event listeners (probably to better track document contents as it changes). You might not want Excel firing an event for every cell you're changing via code, and turning off events will speed up your VBA code performance if there is a COM Add-In listening in on Excel events. (Thanks to Doug Jenkins for pointing this out in my earlier post).
ActiveSheet.DisplayPageBreaks: A good description of this setting already exists:http://support.microsoft.com/kb/199505 (Thanks to David McRitchie for pointing this out).

Read/Write Large Blocks of Cells in a Single Operation
This optimization explicitly reduces the number of times data is transferred between Excel and your code. Instead of looping through cells one at a time and getting or setting a value, do the same operation over the whole range in one line, using an array variable to store values as needed.
For each of the code examples below, I had put random values (not formulas) into cells A1:C10000.
Here's a slow, looping method:
Dim DataRange as Range
Dim Irow as Long
Dim Icol as Integer
Dim MyVar as Double
Set DataRange=Range("A1:C10000")

For Irow=1 to 10000
  For icol=1 to 3
    MyVar=DataRange(Irow,Icol)  'Read values from the Excel grid 30K times
    If MyVar > 0 then 
      MyVar=MyVar*Myvar ' Change the value 
      DataRange(Irow,Icol)=MyVar  'Write values back into the Excel grid 30K times
    End If 
  Next Icol
Next Irow
Here's the fast version of that code:
Dim DataRange As Variant
Dim Irow As Long
Dim Icol As Integer
Dim MyVar As Double
DataRange = Range("A1:C10000").Value ' read all the values at once from the Excel grid, put into an array

For Irow = 1 To 10000
  For Icol = 1 To 3
  MyVar = DataRange(Irow, Icol)
  If MyVar > 0 Then
    MyVar=MyVar*Myvar ' Change the values in the array
    DataRange(Irow, Icol) = MyVar
  End If
Next Icol
Next Irow
Range("A1:C10000").Value = DataRange ' writes all the results back to the range at once
Note: I first learned of this concept by reading a web page by John Walkenbach found here:http://www.dailydoseofexcel.com/archives/2006/12/04/writing-to-a-range-using-vba/
A previous Excel blog entry by Dany Hoter also compares these two methods, along with a selection / offset method as well: http://blogs.msdn.com/excel/archive/2008/10/03/what-is-the-fastest-way-to-scan-a-large-range-in-excel.aspx
...which leads me to my next point.
Avoid Selecting / Activating Objects
Notice that in the above-referenced blog post, the selection method of updating a range was the slowest. This next optimization minimizes how frequently Excel has to respond to the selection changing in the workbook by minimizing the selection changing as much as possible.
Range Example: Again, see the Excel blog post quoted above. It demonstrates that using selection is the slowest of the 3 methods discussed for reading and writing to ranges.
Shapes Example: Setup: I have 40 shapes on a sheet, and I want to write "Hello" in each of them.
Using the slower "selection" method, the code looks like this:
For i = 0 To ActiveSheet.Shapes.Count
   ActiveSheet.Shapes(i).Select
   Selection.Text = "Hello"
Next i
The much faster method is to avoid selection completely and directly reference the shape:
For i = 0 To ActiveSheet.Shapes.Count
   ActiveSheet.Shapes(i).TextEffect.Text = "Hello"
Next i
The concepts illustrated by the examples above can also be applied to objects other than Ranges and Shapes.
Note: I first learned of this concept, in the context of shapes, by reading a web page by Ron de Bruin found here:http://www.rondebruin.nl/shape.htm
Related Performance Paper
See the "Improving Performance in Excel 2007" paper on MSDN: http://msdn.microsoft.com/en-us/library/aa730921.aspx
This is a fairly detailed and comprehensive paper that introduces the bigger grid and increased limits in Excel 2007, and primarily focuses on Excel calculation performance and debugging calculation performance bottlenecks. There's also a short section on how to write faster VBA macros.
Other Performance Optimizations
While the above optimizations are what I consider the most important, there are a few other "honorable mention" optimizations I will mention briefly for you to consider.
Consider the performance gains by implementing your code's functionality via XLL / C-API. An overview and supporting materials for the SDK can be found here: http://msdn.microsoft.com/en-us/library/bb687827.aspx .
Declare variables with explicit types to avoid the overhead of determining the data type (repetitively if used in a loop) during code execution.
For simple functions used by your code in high frequency, implement them yourself in VBA instead of using the WorksheetFunction object.
Use Range.SpecialCells() to scope down the number of cells your code needs to work with.