100Mbps-1000Mbps NICs all limited to ~40Mbps?

I have paid for 1000Mbps for quite some time from my ISP and have always had trouble with it maxing out at ~40Mbps. I always faulted my VPN and just suffered through it. I finally tried to troubleshoot the issue and connected my netbook to the ONT (via ethernet), which has a NIC capable of 100Mbps (per https://askubuntu.com/questions/431911/how-can-i-verify-the-speed-of-my-nic-in-ubuntu/1048051#1048051 ), but I still only got ~40Mbps. So I had the ISP send a technician out, and after much push-back because they wanted to connect to my router rather than use their own tools, they finally agreed

Question by DiogenesOfMiami on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.