Hello,
Have a look at that link:
http://www.smallnetbuilder.com/wireless/wireless-basics/30664-5-ways-to-fix-slow-80211n-speed
The trick is to have only 802.11n network adapters in the network, else when a legacy adapter is introduced in the network , the throughput wil roll back to 54mbps.
Also,
encryption: WPA2/ AES
channel: Try setting on 20/40MhZ (in some location, the 40 MHZ setting can rather lower the troughput,sometimes)
See the link:
http://compnetworking.about.com/od/wireless/f/80211n-300-mbps.htm
Enable WMM in the router
Check your network adapter's driver options in device manager and use n mode only, and channel widh identical to the router (available in Centrino adapter, maybe in others).
I hope you'll be able to solve that matter.
Regards.