I found the culprit: it was Active Protection (active by default) from "Acronis True Image for Western Digital", I switched it off, and the problems ended.
I installed a new WD BLACK SN850X 1TB replacing in slot 1 (PCIe Gen 4) of my DELL Inspiron 16 Plus (7610 ) the original Kioxia KBG40ZNS512G 512GB. The Kioxia got degraded to slot 2 (PCIe Gen 3). To my surprise, CrystalDiskMark 8 has shown that the "best of the best" SN850X performs worse than the budget Kioxia except for the sequential read scores. Worse even, the Inspiron got a very annoying hiccup, stalling randomly here and there for a few or even a few dozen seconds - while attempting to a new app e.g. Not a short research directed me to the DELL BIOS settings where under the Storage tab there is a default choice "RAID on". I followed (important!) the procedure described here: [URL] where I went from here [URL] and voilá: the Intel Rapid Storage controller gave up to the original WD and KIOXIA controllers and both disks went up immediately in their scores. The WD finally showed better scores than KIOXIA (although I expected more gap in the random read/write area, but it seems that since at least 2019's Samsung  970 EVO Plus there is no real breakthrough in the field, so from the user - or gamer - perspective, the faster PCIe Gen 4  drives bring no noticeable gain). Note: when doing a benchmark check disable Windows write-cache buffer flushing in the WD dashboard to get max random write readouts, and disable Windows write-cache buffer altogether to get max sequential write readouts - after the benchmarking revert to the default settings, they seem to be working well in the real world. And most importantly the perceived performance of my machine, which I use to make my living as a WordPress developer, went up significantly - no hiccups anymore (so far). BTW: check if your Windows power plan settings are not set to switch off the disk after a short time - for SSD drives, this is not needed any longer. Not a good idea for DELL to set the BIOS storage settings by default to "RAID on" instead of AHCI/NVMe still in modern times.
New NVMe drive installed and underperforming due to RAID on set in BIOS by 
default