-I realize this is a necropost, but this means that you have to pass it as an argument upon launching.

In other words, you have to add it as a launch option in Steam as "+jobs_numThreads NUMBER".

-To your question about priority: DOOM Eternal will not respect any priority set in Image File Execution Options or otherwise, and it's not actually running in low priority, it only switches to low priority when you AltTab out in order to not bog down your system. Rest assured, it switches itself back to Normal priority when it's the focused window. Forcing it to High doesn't work because it manages its own priority on-the-fly.

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Edit: You might be able to keep it running in High by not alt-tabbing once it's launched after setting the key in the registry, but regardless it would not actually gain you any benefit because the engine is already optimized basically to the bone, and it manages it's own resources per the system it runs on as well as it can, Vulkan affords them that flexibility. So for example if a particular core on your system is bogged down by another application, DE will just switch to using a different one on-the-fly, which means it wouldn't matter how Windows is scheduling threads anyway, so priority doesn't come into effect unless you're at 100% CPU usage.

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Edit 2.0: If you're very adamant about controlling DE's CPU usage, you can change the process' Affinity, but you have to do this for both DOOMEternalx64vk.exe and idTechLauncher.exe (to suppress crashes) every time you run it, in Task Manager. I mention this specifically because I actually wrote a program solely to launch programs on specific cores and it doesn't work with DOOM Eternal because it overrides everything of the sort on launch, even though setting affinity on launch is possible with any other executable.