VMs have huge issues. Even if you get working drivers, it still can fail on all sorts of applications. Just try one thing: Disable GPU accelleration and test, if the CPU at least ramps to 100%. It absolutely should. If not, there might be other driver issues. I even had some problems inside VMs with HDD drivers (yeah, there seems to be some emulation of some sorts in some circumstances). The HDD only ramped up to 2mb/s for large files. Absolutely unusable. It should not matter that much for Gigapixel AI.
Just try CPU only and report, if the usage of CPU ramps up more.
Edit: I know it is verry unlikely, but could it be, that the problem comes from no attached monitor? I even ran into issues with no monitor connected to the GPU (not in Gigapixel, but worth investigation, since it seems GPU related). Also look at harddrive usage. Benchmark of HDD would be also a good idea, to figure out, if there are performance issues. A RAM benchmark also can tell you, if the RAM performs as expected.
All extremely unlikely, but definitely possible. (Compatibility mode tested? Just fall back to Windows 8 or 8.1 for example. Would be just a workaround, if it works - but better than nothing)