We just got 1Gbps fiber internet and it seems like the C5400 can't handle it! I'm guessing there's some tweak that needs to be done, but here is the lowdown (all values given are in Mbps):

Direct connection from modem to PC (bypassing the router) -

943 Up // 897 Down

Modem to router using speedtest on router interface -

402 Up // 140 Down

Modem to router to LAN connected device - 

212 Up // 133 Down

Modem to router to wireless device -

78 Up // 41 Down (similar on all bands)

As you can see, the router throttles the bandwidth by well over half. I have QOS off and app priority set to normal. I've also set a manual max speed of 999Mbps up and down, but there has been no change. 

The cable is good, I'm using the same cable I used between the router and modem that I did between the modem and PC so it's definately the router. 

Can anyone suggest settings that may improve my throughput on LAN and wireless?