After a few hours of troubleshooting on my own network. I took my 5100 to a friends house and was able to get the expected speeds there. So I returned home and called my ISP. They made some changes on their end (changed vlans and ports) and now it is working properly!
The ISP is new to the area, and still has some bumps.
Sorry for the question to the group, but I am good now.
@michmoor - I do use pfBlockerNG - but I had disabled that during troubleshooting. With all re-enabled, I am now getting expected speeds.
I don't have an answer to the obvious question of why did it work with the PC connected directly to the ONT. The only thing I can come up with is that I did reboot the ONT between tests, so that may have had an impact.
Thanks again to the group.