Yeah, that's exactly why I'm just gonna do a standalone bare-bones separate Windows install, see if that makes any difference - if it does, at least then I know it's SOMETHING to do with this install, and I can go through and install stuff one at a time to see what the issue is.
The only leechy-type stuff I have installed are the Thermaltake (which is required to set fan speed, then it can be closed) & Gigabyte RGB softwares, and I close both of those out (and kill them in task manager if they don't close out completely) before benchmarking... neither of those have been updated since long before I even got the Kingpin card, so I doubt they're the problem.
I also don't run much in the way of bloatware... I'm getting rid of Logitech gear, again, slowly but surely, to rid my system of that stuff, even though I doubt it's much of an issue, I don't have any AV software because I don't visit sketchy websites, just use the standard Windows defense stuff - I do have Malwarebytes, but I only have that running when it's running a scan because it's the free version so it doesn't have realtime protection anyway.
I don't use the Steam version of 3DMark, I use the standalone, so Steam isn't causing the issue either. I just don't get it.
The only thing I can think of is like you said, some dodgy Windows update or something FUBARed stuff, and the only way to (hopefully) fix that is a new install.