Thank you for your post. There are a couple of possible causes. One would be an actual issue with the wireless adapter, see if you can connect to another network in N speeds. If you have still trouble the issue would be related directly with the adapter, you would need to have Asus replace it for you.
Bur if the issue does not follow the adapter it could certainly be related with your network. Check on the router some settings, try to use only specific settings for security and encryption, (avoid mixed modes) see also some recommendations for settings on the router and mostly the laptop at the link below:
Thank you very much for your answer : problem solved! The router was configured with wifi N and a wep key. This is not recommended and as explained by the link you provided in your post, the speed is not at its maximum in that configuration. Switching to WPA2-AES did it.
Thank you again,