Hi Patrick,
Additional Paravirtualization Interface multimedia testing to drive this issue to a conclusion.
64-Bit Linux Host running VBox 5.0.10 and running a fully updated Whonix Gateway 11 with
128 Mb allocated DRAM as one of the Guest operating systems
Test 1: Running these two Guests concurrently:
Gateway 11 Paravirtualization Interface = None
Whonix Workstation 11 Paravirtualization Interface = None
Notable Results:
In the Gateway: Notable Observations: whonixcheck works, and the Gateway operates
correctly. I suspect the hrtimer interrupt issue is an anomaly, as I have now seen it reported when whonixcheck is not running. Does not appear to impact anything.
In the Workstation: whonixcheck works, YouTube works, and VLC works - and everything works correctly at ‘normal’ speeds.
+++++++++++++++++++++++++++++++++++++++++++++++++++
Test 2: Running these two Guests concurrently:
Gateway 11 Paravirtualization Interface = None
A 64-Bit Linux Guest OS pointed at the Gateway with Paravirtualization Interface = None
Notable Results:
In the Gateway: Notable Observations: whonixcheck works, and the Gateway operates
correctly.
In the 64-Bit Linux Guest OS: YouTube works, and VLC works - and everything works correctly at ‘normal’ speeds.
+++++++++++++++++++++++++++++++++++++++++++++++++++
These results show, at least on my system, that even with both the Gateway and the other Guest OS using Paravirtualization Interface = None, the overall Whonix user ‘experience’ remains within an acceptable range.
Perhaps this configuration is not optimal, but it is indeed, acceptable. 
CCP