I now have a proper “fix” for this on my laptop rather than a mere “workaround”.
The standard advice is to update the GPU drivers. Mine (for RTX 3060) said driver 27.21.14.6262 dated 11/05/2021, and that it was up to date. It appears that the word of Device Manager can’t be relied on.
The NVIDIA web site provided an update to driver 31.0.15.2849 dated 02/02/2023 (having first set a restore point in case of need).
I did a test of 100 files (all 20MP Lumix RAW RW2 files from two cameras and a mix of lenses) - 4.5 seconds per file none came out black.