I may have fixed your slow speeds!
After downloading an installing the amplifi gaming edition yesterday I too was plauged with 50mbps on all devices with a gig connection and well over 300mbps with my archer c5400. When tooling around with the settings, I enabled the "standards" they suggessted but also updated my DNS to cloudfare's 18.104.22.168 Instantly on all devices and bands I started to see consistently 150+ mbps and up to 250 on some devices.
I do not think this is just a simple cloud flare DNS fix, as after rebooting the speeds went back to 50mbps. So i cleared the dns from 22.214.171.124 to the standard and boom back to 3-5x speeds. I feel that making any change to the DNS without rebooting is fixing my issues.
HARDWARE NAT - Enabled
mdsu - Enabled
DNS - 126.96.36.199
sec dns 188.8.131.52
UPNP - Enabled
NOT IN BRIDGE MODE
Prioritizing " throughput"
Speeds set to 940000kbps and 880000kbps (per fios's max speed claims of 940mb/880mb)
I let the network sit over night and tested again first thing today still getting 2-x5 speeds from yesterday.
Hope this helps