When troubleshooting performance issues with Outlook 2003 attachments (connecting to Exchange 2007), we observed severe latency when opening small files (<1MB). The problem persisted despite:

• Creating new Windows user profiles
• Rebuilding Outlook profiles
• Verifying normal behavior on other workstations

Process Monitor logs showed these critical patterns:

15:23:01.123  OUTLOOK.EXE   FAST I/O DISALLOWED   C:\Users\[user]\AppData\Local\Microsoft\Windows\Temporary Internet Files\
15:23:04.567  OUTLOOK.EXE   IRP_MJ_CREATE         Delay: 3200ms
15:23:07.890  EXPLORER.EXE  FAST I/O DISALLOWED   \\fileserver\share\document.docx

The FAST_IO_DISALLOWED error occurs when:

1. The file system driver rejects fast I/O path requests
2. System falls back to slower IRP (I/O Request Packet) processing
3. This creates significant latency for small, frequent operations

Common triggers include:

Check loaded filter drivers:

Get-WmiObject Win32_SystemDriver | 
Where-Object { $_.PathName -like "*filter*" } | 
Select-Object Name, State, PathName

Monitor I/O patterns:

# Requires Admin rights
Start-Transcript -Path C:\io_debug.log
Get-Counter '\Process(*)\IO Data Operations/sec' -Continuous
Stop-Transcript

Add these values temporarily for diagnostics:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management]
"LargeSystemCache"=dword:00000001
"DisablePagingExecutive"=dword:00000001

For our specific case, these steps resolved the issue:

1. Uninstalled legacy antivirus components
2. Updated network adapter drivers
3. Modified SMB settings via Group Policy:

   Computer Configuration > Policies > Administrative Templates > Network > Lanman Workstation
   "Enable insecure guest logons" = Enabled
   

4. Cleared Outlook cache:

   outlook.exe /cleanviews
   outlook.exe /cleanautocompletecache
   

Confirm resolution with:

# Check for FAST I/O failures
Get-WinEvent -LogName "Microsoft-Windows-Kernel-IO/Operational" | 
Where-Object { $_.Message -like "*FAST_IO_DISALLOWED*" }

When troubleshooting Outlook 2003's painfully slow attachment handling (especially with Exchange 2007 backend), the FAST I/O DISALLOWED error in Process Monitor reveals a fundamental file system bottleneck. This isn't just an Outlook quirk - it manifests across network file operations, though becomes particularly noticeable with email attachments due to Outlook's temp file handling mechanism.

Windows' Fast I/O mechanism bypasses certain IRP (I/O Request Packet) processing for performance gains. When disallowed, the system falls back to traditional (slower) I/O paths. Common triggers include:

// Pseudo-code illustrating I/O path decision
if (FileSystemSupportsFastIO() && !SecurityRestrictions()) {
    UseFastIOPath();
} else {
    FallbackToStandardIRP(); // ← Where delays occur
}

From multiple troubleshooting cases, we've identified these frequent culprits:

• Antivirus Overreach: Real-time scanning intercepting I/O paths
• NTFS Corruption: Particularly in user profile directories
• Group Policy Restrictions: Enterprise security policies disabling fast paths
• Broken Filter Drivers: Legacy or misconfigured storage stack components

Here's a PowerShell script to diagnose I/O path issues:

# Check for filter drivers
Get-WmiObject Win32_SystemDriver | 
Where-Object { $_.Name -like "*flt*" -or $_.Name -like "*filter*" } |
Select-Object Name,State,PathName

# Verify NTFS integrity
Repair-Volume -DriveLetter C -Scan
Repair-Volume -DriveLetter C -SpotFix

# Check for AV interference
(Get-MpPreference).DisableRealtimeMonitoring = $true # Test with protection disabled

Case Study: A financial firm resolved similar issues by:

1. Creating a dedicated Outlook temp directory with specific permissions:

   mkdir C:\OutlookTemp
   icacls C:\OutlookTemp /grant "USERNAME:(OI)(CI)(M)"

2. Forcing Outlook to use this location via registry:

   Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\11.0\Outlook\Security" -Name "OutlookSecureTempFolder" -Value "C:\OutlookTemp"

When the issue manifests across network shares, consider these Wireshark filters to analyze SMB traffic:

smb2.cmd == 0x05 ||  # SMB2 READ
smb2.cmd == 0x08 ||  # SMB2 WRITE
smb2.flags.response == 0

Compare latency between SMB2_READ requests and responses to identify storage-side bottlenecks.

For Windows 7/Server 2008 R2 systems, these registry values often help:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem]
"NtfsDisableLastAccessUpdate"=dword:00000001
"DisableDeleteNotification"=dword:00000001

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\LanmanWorkstation\Parameters]
"DirectoryCacheEntrySizeMax"=dword:00000100
"FileInfoCacheEntrySizeMax"=dword:00000100