I got puzzled last week by my Surface Pro 3, which usually connects at 60-90Mbps to my router, regardless of which band (2.4 or 5Ghz I choose). I knew Surface 3 had some problems with low WiFi connection speeds when it was released, but since then Microsoft has released drivers and firmware updates that were supposed to fix the problem. I upgraded to an AC router, Surface Pro also has AC adapter, so why am I not seeing better speeds?
My WiFi router is a Netgear Nighthawk R8000, which boasts a 3.2Gbps WiFi speed. That's just for PR, in reality, it has one 2.4GHz band with max 600Mbps and two 5GHz channels, each supporting max 1300Mbps. So, the max speed of connection is limited to the max speed of the band I'm using. But, that's not the end of the story - both my Surface Pro 3 and my wife's laptop connect at maximum 866.5Mbps, and that's when staying 2-3 fests apart from the router. The speed is actually negotiated between the router and the client device. If I move 10 feet away, the speed starts dropping to 700Mbps. If I stay in living room, the speed drops to 80-90Mbps.
Surface 3 Pro has a 'Marvell AVASTAR Wireless-AC Network Controller' Wi-Fi adapter, and based on http://www.marvell.com/wireless/avastar/88W8897 it's maximum WiFi speed is 867Mbps.
I'm reaching this speed (so Microsoft kept its promise and fixed the low speed problem), but I have to be feet apart from the router to reach it. And even in these ideal conditions I'd need at least 4 Surfaces to saturate the two 5GHz channels plus more WiFi devices connecting on 2.4GHz to reach the advertised router's speed... The router speeds are just a PR gimmick.
Start by installing Remote Server Administrator Tools For Windows 8.1 to get the 'Remote Server Administration Tools' components, then Turn Windows features on/off, and make sure to select 'AD DS Tools'.