In order to get 802.11n speeds you have to use either Open or WPA2/AES as L2 security. In addition to that you have to enable WMM under WLAN->QoS section. It can be set either "Allow" or "Required"
Keep in mind, if you set it to "Required" then non-WMM capable client cannot join your WLAN. So in your environment if you have mixed client then better to leave that option as default which is "Allow"