Lag, but GPU usage close to 0%

I've tried to downgrade the nVidia drivers to 340 WHQL (the oldest that are required by the game).  It looked better for as far as 30 seconds ... then got back to broken.

I see people complaining about low fps.  On the benchmark I get a minimum of 41fps.  In game, when rotating the camera, I can have as low as one frame every 5 to 13 seconds (with the various hardware parts doing virtually nothing) then go back to high fps for a few seconds.

In the chantry building, underground, there is this room after a small passage light with torches.

When I go from the room back to the stairs, it stutters/pauses at the same spot.

Then I walk a couple of meters and walk backwards : no issue (but it has the exact same rendering to do).

I go forward again : pause. (So what ? Botched up prefetch from the disk ?)

I'm starting to think the game is so bug-ridden that the driver locks from the nonsense the game is sending it (seeing how choppy the loading screen logo animation is, this is a strong contender).

I'm assuming that the source of the game (digital, digital deluxe, disc) is not a factor.

I wonder on what PC machines the game even remotely works.  There must be a commonality. (I have virtually no application running : this is a clean machine for gaming)