You will want to make sure that you have updated your router firmware to the latest version, and check to so do regularly. Asus has quite a few "revisions" to the firmware for these things and they do seem to impact performance quite a bit.
You can choose to use the Asus firmware at:
»support.asus.com/Downloa ··· p=11&s=2
Alternatively there is an open source project DD-WRT firmware available which will give you a lot more advanced options than the Asus default. Depending on how much you want/need to mess with it this may be the way to go.