Iphone 6+ not reaching max wifi speed

I have a iphone 6+ with EE. My fibre optic I supplied by Virgin Media at 120mbps. This speed is almost always reached on www.speedtest.net on my PC. But my iPhone 6+ has only ever got 50mbps max. I have a SuperHub2 and I'm using the 5Ghz connection for wifi on the iPhone. I was under the impression that 802.11ac could reach 120mbps.