There is very little going on in DS at most times which is why it runs so smoothly. The decima engine is not perfect (though it looks great) but to see what it really does to a system you need to play HZD where its implimenting AI, foilage, combat physics and particles, etc, all at once.Yes the GPU is not being fully realised, it's waiting for stuff from the CPU which isn't happening since only one or two cores are being used and the whole performance bottlenecks as a result. Very poorly optimised, even with RT off as you can see from the stats.
On the lfipside I installed Death Stranding director's cut yesterday and marvelled at how well managed the CPU usage is, and as a result the GPU is breathing pretty amazingly. The Decima engine seems supremely optimised and I did some reading up, basically Kojima wanted the PC version to maximise thread usage, so even in scenes where nothing much CPU intensive is going on, the engine would allocate workloads to those idle cores to do other stuff instead thus sharing the work:
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DS and Doom Eternal will forever be my benchmarks for engine optimisation and ludicrous performance.