EDIT: Got an email from my ISP saying "the fiber owner has resolved your issue, we are closing the ticket." I immediately called my ISP out of curiosity, since they earlier had told me that they need to change my ONT for me to get my full speed. Well, it turns out, the fiber owner (don't know the English word for them) can manage speed per port on the ONT. Sic. So for some reason, they had limited the speed to 100 Mbps.
I purchased a 1 Gbps down/up connection and noticed that I was consistently getting 95 Mbit/s down/up, regardless of hardware configuration (router, no router, switch, no switch, connecting directly to the ONT, cat 6/6a cables, etc) and regardless of software configuration (VPN on/off, firewall on/off, OS Linux/Android, driver updates, etc).
When nothing seemed to help on my end, I finally called my ISP. They could confirm that my ONT is a decade old and that they can see that each port only allows for 100 Mbit/s down/up.
I went through these steps before finally testing a direct connection to the ONT which finally made me call the ISP.
The ISP is going to replace the ONT for free.
Specifically B is the standard. Edit: the Wikipedia article has a more complete explanation as to why, but you should always wire to B unless you have an explicit reason to do otherwise.