Your htop screen shot also seems to suggest that htop is seeing only one cpu.
Perhaps htop has a setting to show all cores as one cpu? Not sure
Maybe hold your finger on the purchase of new stuff, I’m not sure this one’s quite case closed just yet 
What’s the output of:
nproc
(should be reporting 12?)
Have you sought a BIOS update for your motherboard? I suspect the most recent update might be from mid 2023, but you’re running a version from mid 2019.
Do you have intel-ucode installed?`
yay -Q intel-ucode
I just solved the problem, and it was all thanks to @Bink’s suggestion. When I typed inxi -Ccsxx --za, I saw that only one core was seen, and I started digging.
My OS was only seeing one core of the CPU, which actually have six cores. I rebooted into BIOS and searched inside the options, just to find out that the Multiprocessing option was disabled. I switched that on, and… well, it was that simple.
Thanks everybody for the efforts, at least now I have an optimized version of the hardware acceleration… (because that needed to be fixed)