Most router firmware use special shortcuts to minimize CPU usage of the router. Broadcom's version of this I believe is called CTF (Cut Through Forwarding). Unfortunately it was designed for IPv4 and on IPv4 is minimizes CPU usage and increases throughput. It seems for IPv6 the CPU has to handle the entire throughput so the IPv6 speed is slower.
Most routers deploy something like CTF. On the Netgear forum there have been several threads about how Netgear routers have been slower on IPv6 speedtest sites than IPv4 but it was not resolved. I am giving a possible answer. I personally have a Netgear RAX45, R6300, and a R7000P.
