I have discovered an interesting bug the other day after several trial and errors to fixed it. Thought I would share this with others out there that are having similar issues. The XB3 modem/router is a leased device provided by Comcast. It has the ability to put the device into Bridged mode to only act as a modem and you can use your own router. The bug had to do with the Netgear routers IPV6 setup not allowing full speeds.
The router used is a Netgear WNDR4300.
After the XB3 put into bridged mode I disconnected the router from it and plugged a computer directly into it via a cat6 cable, then I pulled the power cord off the modem and power-cycled it. After the laptop got the WAN IP facing the internet I did a speed test. It showed full speeds expected. After connecting the router again the speeds dropped and was only showing 1/5th the speeds. After poking around in the router trying to find the culprit I disabled ipv6 and went just ipv4 in the router, I did a speed test and everything was showing full speed. Enabled DHCP ipv6 again speeds dropped. DHCP is the proper setting needed to get ipv6 working. I set the setting to auto-detect and by itself it itself selected DHCP, but the speeds started showing full speeds like it should have.
So if you manually select DHCP you will get issues if you allow the modem to auto-detect (DHCP) it's fine. So... this tells me that there is a coding issue apparently between the manually selection of DHCP versus allowing the modem to select DHCP itself via auto-detect. It seems to be a coding error that is not allowing full duplex 1G speeds on the wire. It might be a isolated problem for this particular Netgear router model.
However, programmers tend to cut and paste code so it could be across a wide range of Netgear router models. If you aren't getting full speeds and are not sure if it's the modem or the router, just connect a computer directly to the modem with a cat6 cable and power-cycle the modem. Be sure that the network card on the computer is set to DHCP. Once the modem is back online with all lights on, your computers IP should have the WAN IP assigned from Comcast. If you see full speeds connected direct and slow speeds with the router, then you found the router is the culprit and likely the same issue with ipv6. Comcast's Xfinity speeds website tests both ipv4 and ipv6, so if there is a issue it will reflect on the speeds mostly. Maybe not so much for lower speed packages, but it will reflect on speeds beyond 150mbit. This is the importance of having modem(docsis 3),router 1G on all ports including 1G Wan port, cat6 wire, network cards supporting 10/100/1000 full duplex, AC wifi network cards that support 10/100/1000. Everything on the network must be 1G for higher tier speeds including the wire itself. Those are the biggest issues that I find. I hope this helps others out there.