Primeiro, achei isso ...
Most 802.11n products will knock your throughput down by up to 80% if you use WEP or WPA/TKIP security. The reason is that the 802.11n spec states that the high throughput rates (link rates above 54 Mbps) can't be enabled if either of those outdated security methods are used.
Significado ...
Use WPA2/AES and Enable WMM. WMM must be enabled in order to get link rates higher than 54 Mbps.
Pesquisei isso aqui em SmallNetBuilder .
Então, mude para o WPA2 e habilite o WMM. Isso deve retroceder suas velocidades entre os clientes N e o roteador N.