Slow shutdowns of Windows 10 on VirtualBox

I'm having extremely slow shutdowns of Windows 10 under VirtualBox. Occasionally the shutdown is within "normal" parameters (as compared to Windows 10 ruing natively on an i5 Dell laptop), but typically they take 10 to 20 minutes. Updates are not occurring as far as I know (no "Update and Shutdown" menu item nor update progress indicators).

Host Settings

• MacBook Pro (Retina, 15-inch, Mid 2015), 2.5 GHz Intel Core i7 4x Core, 16 GB RAM
• OS X 10.11.3 (El Cap)
• VirtualBox 5.0.14 r105127

Client + VB Settings

• Windows 10 Enterprise x64
• 4 cores (same issue with 1 core and 2 cores)
• 6GB RAM
• Video Memory 256MB (yes, tweaked - and yes, I've tried moving it down to 64GB with the same result)
• 3D acceleration enabled (also occurs with it disabled)
• Guest Additions installed

The virtual drive is an 80GB VDI with 26GB free. Other details available upon request (for example, Visual Studio Enterprise 2016 is installed with Azure development enabled).

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