I had my own decent into madness this spring.

I slowly replaced my home network piece by piece trying to find the bottleneck that was causing my gigabit internet to top out at ~300kbps in my office on the other side of the house from the modem.

After replacing the Ethernet cable run from the second floor to the basement with fiber optic... And the switches in between... And seeing no improvement... I tried a different computer with a built-in ethernet port on the same cable, and pulled 920kbps.

The problem... Was my Caldigit Thunderbolt Dock. I replaced it with an OWC one from Facebook marketplace for cheap and it solved the problem... Roughly $500 in. I'm still angry I didn't check that early earlier.

My network is 10 gigabit now though.