As you have exhausted the driver issue I would then attempt the Ethernet test. You never know, someone nearby could of introduced something causing wifi disturbance at a similar time. The laptop you have is more robust and can be moved. Check your router setitngs to ensure nothing changed and/or try a wifi channel change - it could be that something is interfering at the point of your desktop PC. I very much doubt this will help but try disabling anti-virus/cloud protection and see if speed is still slow, if that fails then try creating a new user account to see if the issue exists there. I do sympathise as this sounds an awful issue and of course fully re-installing windows is extremely unpalatable. After that you could try a repair install/in place upgrade of windows although tread wisely, I've never done this before but an in place upgrade should leave all your software that you have installed intact. If it's still slow after all this then you can safely eliminate the card/USB dongle being the problem and blame the bug-fest that is Windows 10.
Finally try and see if there is a BIOS update available for your motherboard. Of course the further away you are the slower the throughput it is but at least it'll be consistent (much like a standard Ethernet connection). Or if that is really not an option try and get a couple of Ethernet plugs to run through the electrics. I know it's a PITA but if you could just temporarily carry the PC / monitor near your router for a quick test.
If anyone has any suggestions I'm very open to them.
For some reason the card only has access to 802.11b speeds despite going up to 802.11n. I don't expect much but it's worth a try. I've also dropped an email in at the customer support for my WiFi card. I didn't feel like attempting this today but I might give it a try tomorrow.
When I installed my SSD I formatted the HDD but when I boot up Windows 10 it still sees the HDD as being bootable media. I have had the suggestion to unplug my HDD to see if there's a difference. Both the USB adapter and the pci adapter are by TP Link. The speeds were fine there so I'm very baffled by what the issue is on my desktop. I also took the USB WiFi adapter I tested against my pci adapter and plugged it into my Windows 10 laptop to test it's speeds there.
I installed some drivers with no results and did a system restore to January 18th to see if it was a recent thing (also no results). So I've been tinkering most of the day with no luck.