RT-N66U w/ Shibby 1.32 cutting Internet speed in half

Hey all,

Cox recently upped my Internet speed to 300mbps.

When I connect directly from a PC to the cable modem, I get the full 300 (and then some).

However, once I put my router right behind the modem, all my speed tests get knocked in half to 150mbps.

I tried removing everything else from the router and shutting off wifi and then ONLY connecting my PC to the router to eliminate all other variables and the problem remains.

This is an ASUS RT-N66U running the latest Shibby firmware.

Any...

RT-N66U w/ Shibby 1.32 cutting Internet speed in half