Internet slow to unusable on PC only (less than 1% of network capacity)

As stated above. I pay for very good internet. The highest home package AT&T Fiber offers in my area (\~500 up/500 down). I have a laptop and a PC I built myself. Both are \~3 years old. The laptop gets 500/500 Mbps over wifi. My desktop gets 3/0 Mbps on wired (the upload speed is so low, the test fails. \~0.05 Mbps). My desktop speeds do not change whether it is wireless or wired... plugged in through my network card or into the mobo itself... after restarting, unplugging, reinstalling, troubleshooting, etc etc.

I don't need crazy speed, but a 100x difference in speed on devices on the same network is crazy and I have no idea where the bottleneck could be. I've tried wifi/wired, different cables, different input. I have no idea where to start and I'm a hairs breath away from hiring a network engineer to come to my house and figure it out.

Any help? Any place to start? My desktop is nearly unusable.