Incredibly confusing network/gaming issue

[Joe Milton]

I used to own a pair of "ethernet homeplugs" for making a wired connection to my pc upstairs whilst my router is downstairs. Unfortunately these broke recently so I'm waiting for a replacement. Whilst waiting for my replacement I cannot move my pc downstairs so I am using usb tethering through my phone (LG G3). The issue is my computer slows down heavily whenever I am tethering. I can run Borderlands Pre Sequel at 140+fps when untethered, I can't even get to the main menu without it pausing every 2 seconds while tethering. Even games with no online options such as Resident Evil grinds to a hault.
Any suggestions on what is causing this strange issue is appreciated

 

[R4lNM4N]

Tethered connections to a mobile network are INCREDIBLY slow. In the meanwhile, you can get a USB wireless adapter and connect to WiFi

 

[Joe Milton]

I understand that my network speed will be very slow, but surely my WHOLE PC experience should not be effected by this?

 

[R4lNM4N]

Unrelated. Have you rebooted yet? Installed the latest video card driver for your.... (Fill in the blank with system specs please)

 

[Joe Milton]

Yeah, I've done all the usual typical things, rebooted it, unplugged my usb, tried all my spare usbs, different cables, removed gpu and put it back in - same for ram, latest drivers for everything, including os
i5-4670k (No OC)
GTX 660Ti
8GB ram
128gb ssd, 1tb hdd, 320gb hdd

 

[R4lNM4N]

Version/edition of Windows?