The CPU stress stalls that some people (including me) experience with stress tests like ITB AVX are even worse than I thought. Up to now I mostly only put time into learning about Ryzen's general operation, compatibility and overclocking, but now I checked DAWBench in Reaper running on a 1800X (Asus Crosshair 6 Hero).
Turns out that even a single logical core being fully (over)utilized by Reaper leads to extreme system stalls close to making the PC unusable. That's 15 logical cores being unused and you still can watch graphic elements being built up from top to bottom and input (keyboard/mouse) being heavily broken. Even processes set to run at Realtime (31) priority get interrupted, all the while neither DPC or interrupt latencies can be measured to increase.
I uninstalled HWinfo and did not run any other software that reads out temps and such. On top of that I did a Clear CMOS and replaced the NVidia GPU driver with the Microsoft standard one. I tried various CPU core affinities and power schemes, different memory frequency and timing settings, all to no avail. My 16 thread CPU is literally turned into a 1 thread CPU for this kind of workload.
This also happens in Safe Mode and without using an actual audio driver (null device). It's worse than what I have seen from running ITB and ITB can even run smoothly without stalls under the same circumstances. To make sure that this is not an issue of Reaper (DAW software) I ran my test on an 8 year old 4-threads laptop and there it worked as expected, aka the software stalled when the core was overloaded while the rest of Windows remained usable.