I have a FiOS G1100 (hw ver 1.03, software 02.02.00.16). Because FiOS doesn't offer IPv6 in my area, I am using a GRE tunnel to carry IPv6 over my IPv4-only network. I have gig service, but can't get anywhere near full performance via GRE. Here's the setup: Tunnel router A is in a datacenter, with a public IP address and a 10G connection. It has a GRE tunnel with the remote side being my external FiOS IP. My G1100 has a DMZ host configured that forwards traffic to the internal (192.168.1.x) address of my tunnel router, Tunnel router B. Tunnel router B has a GRE tunnel configured with the remote side being tunnel router A's external address, and the source being its internal (192.168.1.x) address. This means that all GRE traffic gets natted by the G1100 so that the internal IP becomes external and vice versa. I run iperf testing between the two devices that terminate my tunnel. If I do it via IPv4, I get 800+mbps throughput. If I do it via IPv6 (over the tunnel) I get 150mbps. In both cases, it's using the NAT on the G1100, so it's definitely capable of transferring traffic between these devices at good speed, but when GRE comes into play, the performance suffers. Same results if I do an xfinity speed test (it's dual-stack so I can test with IPv4 and IPv6 and see the difference in performance). I have tested this several different ways, and if I force 700mbps of UDP across the GRE tunnel with IPerf, based on the bandwidth monitors available on the G1100's UI, I can see the traffic come into the G1100 at 700+mbps, but only 150mbps of that arrives inbound at the tunnel box on the other side of the G1100. I suspect that there is some sort of rate-limit on GRE traffic through the G1100 that is affecting me, but I can't find any place to check this in the FiOS router UI. There are no alerts in the firewall logs indicating it's seeing this as attack traffic, either.
Just guessing here, but this could be because of GRE ALG kicking in on the G1100. This would cause the router to send your GRE traffic through the CPU instead of through the hardware flow accelerator in some cases. A lot of home routers don't have CPUs powerful enough to route more than a couple hundred Megabit a second in software.