I just upgraded to a RT-AC56U from a RT-N16 (I had been running Merlin on it until Merlin dropped support for it, then I was running Tomato Shibby/Advanced Tomato). After getting the AC56U set up on the latest Merlin FW as soon as I got it out of the box and configuring everything, I'm noticing that, while my download speeds on the Comcast speed test for IPv4 is coming in at my provisioned speed (I'm on Comcast), my IPv6 download speed is roughly half that, but upload speeds for IPv4 and...
RT-AC56U - Slow IPv6 speeds
RT-AC56U - Slow IPv6 speeds