Problem: Excel opened the first file quickly, but the second and subsequent files very slowly

Problem:
Excel opened the first file quickly, but the second and subsequent files opened very slowly when launched externally (double click from Explorer or using excel.exe "file.xlsx").

Environment:

• Windows 11 Pro 25H2
• Office 2021 Pro Plus
• Clean Windows installation
• Clean Office installation
• Modern hardware (Intel 13th gen + NVIDIA RTX)

Symptoms:

• First Excel file opened fast
• Second and subsequent files became progressively slower
• Opening files from inside Excel was always fast
• Drag & drop from Explorer into an existing Excel window was fast
• Word did NOT have the problem
• Reinstalling Office did not solve it
• Disabling Defender, Protected View, StartAllBack etc. did not solve it

Cause:
Registry DDE handler related to Excel shell open behavior.

Registry key:
HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\Open\ddeexec

The (Default) value was empty.

Fix:
Set (Default) to:

[open("%1")]

Then reboot Windows.

After that:

• multiple Explorer launches became instant again
• no more progressive slowdown
• Excel external opening worked normally again

Important note:
On other healthy machines the value may still appear empty, so the fix may actually force Windows/Excel to rebuild or refresh the COM/DDE association/cache rather than the value itself being universally required.