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 so many stupid steps before finally testing a direct connection to the ONT which finally made me call the ISP. I guess that is the difference between being an educated network engineer and a self taught enthusiast… 💔
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.