Is the speed of your 802.11n Wi-Fi network unexpectedly slow—more akin to the rate you used to get with a 802.11g network? There are numerous possible causes. Most of them boil down to poor signal strength—due to causes such as your Internet device being too distant from the Wi-Fi router or interference from a cordless phone.
However, if you are getting a strong signal and everything else seems to be working fine, your type of password may be the reason behind the slow down.
While this is not exactly a new discovery, Apple chose to highlight it in a detailed support article just posted Thursday. The article explains that, if you use a WEP or WPA (TKIP) password, the maximum transfer rate will be 54 Mbps, even on an otherwise properly configured 802.11n network. As noted here, the maximum possible speed with an 802.11n network is about 300 Mbps, although realistically you are more likely to get in the range of 130 Mbps.
To get the maximum possible speed, choose a WPA2 Personal password. If you have an AirPort Base Station, your only password choices for 802.11n networks are WPA/WPA2 Personal and WPA2 Personal. The latter option is the recommended one. If you have more than one device connected to the router, the former option can still result in slower speeds.
To view or edit your password type on an AirPort Base Station, launch AirPort Utility and open the Base Station’s settings. Next, go to the pop-up menu at AirPort -> Wireless -> Wireless Security.