In addition, the reason why Application.Screenupdating=False helps "optimize"
the performance  of the underlying VBA code,
is because there are no refreshing activities being applied to the screen.
Another statement  in Excel's environment  to help
"optimize" the execution of the underlying code is
Application.Calculation=False, however it is VERY difficult in knowing
when & where to precisely place this statement, because in NOT knowing exactly
when and which specific Excel formulas
in a spreadsheet  ARE being executed at the precise moment within an underlying
VBA subroutine is being executed...

Also think of it this way, when doing ( Application.Screenupdating=False ),
it does several things, it turns off everything in [Excel's] environment  to
"optimize"
the performance  execution of the underlying VBA code.
PLUS, it also delays any updates  to the screen; thus the "flickering/repainting"
and of any refreshing activities in [Excel's] environment. The same is true
for (Application.Screenupdating=True), as it does the reverse of the above.

My thinking is, anything with Application.( ? ) with only one level reference to
[Excel's] environment.
Just like ( Application.Quit ), which is one level reference and it affects all
of [Excel's] environment.

Examples of a specific events that does NOT affect the whole Excel environment,
are Worksheets("Sheet1").Activate & Worksheets("Chart1").Activate.

Shown below are the two WINAPI calls  when the
Application.Screenupdating=True is issued within any VBA application.

Again, don't know if this ia applicable but have you tried a
DoEvents?

Please explain further with a sample, and
what it( DoEvents ) might accomplished.

You seem to be much more accomplished but
I'll give it a go.

DoEvents is a keyword that lets the processing catch up with the code so to
speak.

For example, since we were talking about the VBE... I tried a while back to
build a comprehensive Cross reference for VBE code. I went through all the
code, picked up all the variables/function names/sub names and so on and
then went through the code again to collect references to that stuff. I
collected line  numbers where referenced and defined, module names and
procedure names where referenced, counted the number of times referenced and
so on.

While all this was going on... I've always been of the opinion that you
should give the user something  to see for her money... so I updated labels
in a userform  that reflected what was going on... Looking at this procedure
out of total ... counted so many variables so far this procedure.. this
module..this project etc.

I found that the labels weren't being updated because the code was executing
so fast the label didn't have time  to update  properly... it was just whited
out... Until the end.

I fixed by putting a DoEventsstatement after each of the label updates. It
slowed the process down but you could see what was going on because the
application waited until it's command  stack was empty before continuing.

Hmmmmm... one day I'll go back to that app. I stopped because I found
someone had done something similar... not so comprehensive but it did
produce a cross reference... with *much* less complex code. He'd taken about
2 weeks and I'd taken 5 months over it till then!!! So I thought buggerit.
Ah Well!

But for *intermittent* problems.. I usually look at timing probs first and
spatter my code with DoEvents deleting them judiciously till I get the
culprit. That's worked well when it's worked.. but it's not applicable to
lots of stuff and as I said... maybe not for you.

It's just a statement... It's in the help even... but if you still want an
example I'll send a bit of code from that application.

I just think *full* cross references are soooo valuable. You can see where
things aren't referenced and so on. I went on to start deleting variables
that weren't referenced, sorting Dim statements in procs and all kinda good
stuff... like going through code and deleting stuff commented as debug etc
etc. I got zapped though when I started deleting code from the same module I
was running. Which leads me to suspect that VBA looks down the code evertime
for the next line tag rather than following through.

And here is a set  of procedures to place in a module which could be named as (
mEnvironment ).
It also allows you to protect the environment  of your workbook, from accessing
the sensitive data  in it.
By calling the respective procedures in the proper places within your workbook.

Option Explicit
'===============================================================================\
================================
'-Environment Settings EndUser set initially.
'===============================================================================\
================================
Public StdCMDBar As Integer, FrmCMDBar As Integer
Public DspSTABAR As Integer, DspWINTBar As Integer, DspFRMLBar As Integer
'//=====================================================================
'// Run DisableGettingIntoVBE from an Event procedure
'// eg. Workbook_Open or Worksheet_Activate
'// THEN run EnableGettingIntoVBE from the Opp Event procedure
'// eg. Workbook_Close or Worksheet_Deactivate
'//=====================================================================
'// Note:
'// In order to Disable access into the VBA Editor
'// you must Disable ALL references to Access the code
'// eg Macros...dialog, View Code - available via RighClick on sheet  Tab
'// Record New Macro..., Design Mode as the User can put the
'// workbook in design mode then select a control & double click to
'// view code, right click top  Document Area etc
'// Also you need to Disable the Custom Toolbar List AND
'// the fact that Double clicking ANY area of the commandbars will
'// also give you the Customize Toolbars Option Dialog.
'// The following Routine Takes care of this.
Const dCustomize As Double = 797
Const dVbEditor As Double = 1695
Const dMacros As Double = 186
Const dRecordNewMacro As Double = 184
Const dViewCode As Double = 1561
Const dDesignMode As Double = 1605
Const dAssignMacro As Double = 859

'===============================================================================\
================================
Sub M_ENVRN(Restore As Integer)
If Restore = 1 Then
'====================================
'Reset previous Environment Settings
'====================================
On Error Resume Next
Application.CommandBars("Standard").Visible = StdCMDBar
Application.CommandBars("Formatting").Visible = FrmCMDBar
Application.DisplayStatusBar = DspSTABAR
Application.DisplayFormulaBar = DspFRMLBar
Application.CommandBars("Worksheet Menu Bar").Reset
Application.CommandBars("Worksheet Menu Bar").Enabled = True
Application.CommandBars("Worksheet Menu Bar").Visible = True
'======================================
'-Find out what Version MSOffice
'-is installed.
'======================================
If Int(Val(Application.Version)) > 8 Then
'Excel 2000 and the later version
Application.ShowWindowsInTaskbar = DspWINTBar
Else
'Excel 97
'==============================================
'-Not valid for Office97
'Application.ShowWindowsInTaskbar = DspWINTBar
'==============================================
End If
Err.Clear
'==========================================
'-This setting is only when to not show
'-other new workbooks  to be displayed
'-within the application, to not be
'-displayed on the taskbar.
'(Application.ShowWindowsInTaskbar = False)
'(Application.ShowWindowsInTaskbar = True)
'==========================================
Else
On Error Resume Next
'====================================
'Save current  Environment Settings
'====================================
StdCMDBar = Application.CommandBars("Standard").Visible
FrmCMDBar = Application.CommandBars("Formatting").Visible
DspSTABAR = Application.DisplayStatusBar
'===============================================
'-Not valid for Office97
'DspWINTBar = Application.ShowWindowsInTaskbar
'===============================================
DspFRMLBar = Application.DisplayFormulaBar
'======================================
'-Find out what Version MSOffice
'-is installed.
'======================================
If Int(Val(Application.Version)) > 8 Then
'Excel 2000 and the later version
DspWINTBar = Application.ShowWindowsInTaskbar
Else
'Excel 97
'==============================================
'-Not valid for Office97
'DspWINTBar = Application.ShowWindowsInTaskbar
'==============================================
End If
Err.Clear
'==========================================
'-This setting is only when to not show
'-other new workbooks to be displayed
'-within the application, to not be
'-displayed on the taskbar.
'(Application.ShowWindowsInTaskbar = False)
'(Application.ShowWindowsInTaskbar = True)
'==========================================
End If
End Sub

Function M_CmdBarReset(MainSheet As String, Reset As Integer, SheetName As
String)
On Error Resume Next
If SheetName <> MainSheet Then ActiveWorkbook.Sheets(SheetName).Visible =
False
Sheets(SheetName).Activate
If Reset = 0 Then
Application.CommandBars("Chart Menu Bar").Reset
Application.CommandBars("Chart Menu Bar").Visible = True
Application.CommandBars("Chart Menu Bar").Enabled = True
Application.CommandBars("Chart Menu Bar").Controls("Tools").Enabled =
True
Application.CommandBars("Chart").Visible = False
Application.ActiveChart.ProtectSelection = False
Application.ActiveChart.ProtectFormatting = False
Application.ActiveChart.ProtectData = False
Application.ActiveChart.Unprotect
Application.CommandBars.ActiveMenuBar.Reset
Application.CommandBars.ActiveMenuBar.Visible = True
Application.CommandBars.ActiveMenuBar.Enabled = True
Application.CommandBars("Worksheet Menu Bar").Reset
Application.CommandBars("Worksheet Menu Bar").Enabled = True
Application.CommandBars("Worksheet Menu Bar").Visible = True
Application.CommandBars("Chart Menu Bar").Protection = msoBarNoCustomize
Else
Application.CommandBars("Chart Menu Bar").Reset
Application.CommandBars("Chart Menu Bar").Visible = False
Application.CommandBars("Chart Menu Bar").Enabled = False
Application.CommandBars("Chart Menu Bar").Controls("Tools").Enabled =
False
Application.CommandBars("Chart").Visible = False
Application.ActiveChart.ProtectSelection = True
Application.ActiveChart.ProtectFormatting = True
Application.ActiveChart.ProtectData = True
Application.ActiveChart.Protect
Application.CommandBars.ActiveMenuBar.Reset
Application.CommandBars.ActiveMenuBar.Visible = False
Application.CommandBars.ActiveMenuBar.Enabled = False
Application.CommandBars("Worksheet Menu Bar").Reset
Application.CommandBars("Worksheet Menu Bar").Enabled = False
Application.CommandBars("Worksheet Menu Bar").Visible = False
Application.CommandBars("Chart Menu Bar").Protection = msoBarNoCustomize
Application.CommandBars("Toolbar List").Enabled = False
End If
Err.Clear
End Function

Sub Dummy()
'// NoGo, leave blank OR Display a message eg.
10 'MsgBox "Sorry this command  is NOT available", vbCritical
End Sub

Sub DisableGettingIntoVBE()
Application.VBE.MainWindow.Visible = False '// Close ALL VBE Windows 1st!
CmdControl dCustomize, False '// Customize
CmdControl dVbEditor, False '// &Visual Basic Editor
CmdControl dMacros, False '// Macros...
CmdControl dRecordNewMacro, False '// Record New Macro...
CmdControl dViewCode, False '// View Code
CmdControl dDesignMode, False '// Design Mode
CmdControl dAssignMacro, False '// Assig&n Macro...
Application.OnDoubleClick = "Dummy"
Application.CommandBars("ToolBar List").Enabled = False
Application.OnKey "%{F11}", "Dummy"
End Sub

Sub EnableGettingIntoVBE()
CmdControl dCustomize, True '// Customize
CmdControl dVbEditor, True '// &Visual Basic Editor
CmdControl dMacros, True '// Macros...
CmdControl dRecordNewMacro, True '// Record New Macro...
CmdControl dViewCode, True '// View Code
CmdControl dDesignMode, True '// Design Mode
CmdControl dAssignMacro, True '// Assig&n Macro...
Application.OnDoubleClick = vbNullString
Application.CommandBars("ToolBar List").Enabled = True
Application.OnKey "%{F11}"
End Sub

'===============================================================================\
=================================
'-The following code is useful if you don't want a user to be able to edit the
'-Excel's main  menu/commandbar by right clicking on it.
'===============================================================================\
=================================
'Purpose : Enables or disables Excel's main menu commandbar
'Inputs : [bEnable] If True enables the Excel's right click
menu, else disables it
'Outputs : N/A
'Notes :
'===============================================================================\
================================================
Function CommandBarRightClick(Optional bEnable As Boolean = False) As Boolean
On Error GoTo ErrFailed
CommandBars("toolbar List").Enabled = bEnable
CommandBarRightClick = True
Exit Function
ErrFailed:
Debug.Print "Error in CommandBarRightClick: " & Err.Description
CommandBarRightClick = False
End Function