I've found a solution for me, maybe it works for somebody here. Thanks for all your contributions, I tried everything that was posted here so far, but something else helped me.
I've been for a few days trying and at some point gave up. I even updated motherboard BIOS and VBIOS. I have also tried older drivers for my network card (Killer). However, I have stumbled upon a windows service "Killer Dynamic Bandwith Optimization" and some other Killer related services. They may have been installed and half-uninstalled in the past. Just killing the all the Killer related Windows Services with Task Manager helped. They were 4 in total. This did the job for me.
If it works for you, you can edit the services to they don't start up again after reboot, because on my case there was no way to uninstall them.
I hope this helps somebody in the future.
p.s.: one thing that indicated my computer was fine was that a Linux running on a VM on the Windows machine could download at very high speeds, but Windows (the host) would be capped at around 1.5mb/s.