Esse é um problema conhecido: link (engraçado o bastante aconteceu de vê-lo aparecer recentemente no feed RSS da base de conhecimento)
If an application is preventing Windows from shutting down (ex. unsaved work), Windows will wait a period of 60 seconds at the force shut down dialog before forcing the application to close so shut down can continue. If the user cancels out of the force shutdown dialog, Windows still retains the 60 second timeout value. As a result, the next time a shut down occurs, even if there are no blocking applications, shut down will be delayed until the timeout value expires.
This behavior only occurs when configuring the power button to shut down the machine. When shutting down Windows through the Start Menu, the problem does not occur as the timeout value is not retained when cancelling out of the forced shutdown dialog.
Infelizmente, a solução alternativa é:
To work around this issue, use the shut down option from the Windows Start Menu instead.