Thanks very much for your thoughts, Crispy81. Much appreciated.
Just to confirm - (i) am using Steam, not GOG, (ii) have posted both files to the POE folder, i.e., C:\Program Files (x86)\Steam\SteamApps\common\Pillars of Eternity (not POE2, which I don't have) and (iii) removed the original files I downloaded a week or so ago in case they may be corrupted and downloaded them again today. Unfortunately, I like Harry.agamez, pressing ctrl + shift + backspace does nothing whatsoever.
I'm at complete loss as to what to do. Unfortunately, the Obsidian support staff - normally so helpful - appeared now to have washed their hands of me/this problem (ctrl + shift + backspace doing nothing).
I'm now at my wits end and am on the brink of abandoning PoE as unplayable (or at least unplayable without something like a hallucinagenic to offset the blood pressure-raising effects of the glacial gamespeed). So I wonder whether you, or anyone else, might have any thoughts on a way through or any other way to reduce the CPU cores?