I have bought dell r15 5537 laptop with intel wireless-n 7260 and I faced same problem with slow speed (router is Asus RT-N15U). But funny is that after 2 days of fighting with windows 7, windows 8.1 dozen of drivers - I found the culprit in my case.
When laptop lid is closed - the speed is 1-8 mbit. When it is open - everything's ok.
(I use external monitor, keyboard and mouse).
This laptop has aluminium lid. Just for test I wrapped opened lid with foil. The same result. 1-8 mbit. So the problem in my case was closed lid where is placed wi-fi antenna.
I have installed latest driver via intel update tool (Wireless_17.13.11_s64.exe)