After some further digging, I have discovered a solution to the VR issue. Some fine folks over at Flight Simulator Forums happened across a workaround that I can confirm works like a dream. It has something to do with performance monitoring within Windows.
Here's the solution for uncharacteristically poor VR framerates:
Right-click on your Start button and select "Computer Management". Go to System Tools → Performance → Data Collector Sets → Event Trace Sessions.
Look for the "HolographicShell" process, select it with a right click and choose "Stop".
Apparently, it's a bug that is causing an infinite loop. kind of like Windows is DDoSing your VR headset display.
This has been known for quite a while, so why Microsoft haven't addressed it with a hotfix is beyond me!
I hope this helps someone.