AirPort: Slow 802.11n connection speed when connected using older WEP or WPA security mechanisms（情報元のブックマーク数）

802.11nでWEPやWPAを使うと速度低下を起こすとの事。

You may notice that if you have wirelessly connected your 802.11n-capable computer or other device to your 802.11n router or Wi-Fi Base Station that your data rate will not go above 54 Mbps (megabits per second) despite the fact that you appear to have a strong signal. This should only occur if your router or Wi-Fi Base Station password authentication type is WEP or WPA.

AirPort: Slow 802.11n connection speed when connected using older WEP or WPA security mechanisms - Apple Support

TKIPとWEPだと最大54Mbpsとのこと

This behavior is normal. It happens because the maximum transfer rate for 802.11n using WEP or WPA (TKIP) authentication is 54 Mbps, as specified in the IEEE 802.11n standard.

WPA2使ってね。

To allow the maximum 802.11n rates, ensure that your router or Wi-Fi Base Station is configured to use WPA2 or, alternatively, no password. For security purposes, it is recommended that you select WPA2 Personal when configuring your Wi-Fi Base Station.

AirPort: Slow 802.11n connection speed when connected using older WEP or WPA security mechanisms - Apple Support