I don't have direct experience with this issue with Bitcoin Core, but:
* In my experience with various other CPU intensive software I use (more than average) max - 1 usually leaves Windows quite responsive.
That's set by default, yet, he mentioned that the PC isn't responding while Bitcoin Core is using all of his Cores.
That depends on the specs and the PC's usage,
-par is only for script verifications and if all set cores are fully utilized, the remaining core will be used by Bitcoin Core's other processes.
-snip- use -1 or so and you'll see a huge difference (from bad to worse)
You mean
-par=-1?
The suggestion to use a lower value is to keep a few CPU Cores free to solve his 100% CPU usage by Bitcoin Core.
It will reduce Bitcoin Core's sync performance, yes; but that's a solution if his whole PC is not responding due to the mentioned 100% CPU usage.
The PC is pretty powerful with 8 cores, 40Gb RAM and SSD throughout. The blockchain is on its own disk.
This should've been written beforehand.
Whole datadir and not just the blockchain?
If so, you can rule-out 100% system disk usage scenario, the OS shouldn't be affected in this case.
8-cores but still 100% used by Bitcoin Core, low-power low-frequency CPU perhaps?
Or it's actually Bitcoin Core that's not responding, not the PC? Because this is normal in Windows.