I recently moved house and had Virgin install their 50mb broadband. They fitted the newer SuperHub 2 router which emits at both 2.4 and 5Ghz. All devices connected work fine and get around 28 to 48mb over wifi depending on location in the house via speedtest.net.
The one exception is my Nexus 5 phone. The connection is a consistent 3mb using the speedtest.net app. I have a work iPhone 5S sat right next to it and it benches 48mb using the same app! Strangely, if I disconnect and then reconnect the wifi on my phone by flicking it on and off in settings, it then comes back at full speed, before going back down to 3mb after a leaving my phone for a few minutes.
Any suggestions welcome as its seriously frustrating to see it buffer all the time on youtube etc?! I wondered whether it might be some power saving issue on my phones wifi?