This one came up today with a customer who was extending her Wireless N home wifi network with a wireless access point. The access point was reporting a Wireless N connection at a speed of 54Mb/s rather than 150 or 300Mb/s, and transfer speeds for anything connected to the access point were only 2-3MB/s (not unusual for 54Mb/s). The problem in this case ended up being that the access point was set to TKIP encryption rather than AES – using either WEP or WPA-TKIP may drop speeds back to Wireless G speed, which is 54Mb/s max.
Switching the encryption to AES put speeds back to what they should be. This won’t always be the solution but it’s worth checking if you’re seeing that issue.