What you observe is exactly what I expect to happen. HypterThreading should increase the system performance. That’s what it’s build for!
If I looked it up correctly the 9920X has 12 cores. Therefore you’ll not get into additional real-time thread management because of to the Windows 10 MMCSS limit… and that could theoretically be the explanation of a different system behavior with Windows 10 and multi-core (14+ cores).
As soon as I find time, I will deactivate 4 cores, run some tests and verify this theory.