What Firmware version is currently loaded? Found on the routers web page under status.
What region are you located?
Has a Factory Reset been performed?
Was a Factory Reset performed before and after any firmware updates then set up from scratch?
Was the router working before any firmware updates?
Internet Service Provider and Modem Configurations
What ISP Modem service link speeds UP and Down do you have?
Check cable between Modem and Router, swap out to be sure. Link>http://en.wikipedia.org/wiki/CAT6 is recommended.
Check ISP MTU requirements, Cable is usually 1500, DSL is around 1492 down to 1472. Call the ISP and ask.
I recommend that you have your ISP check the cabling going to the ISP modem, check signal levels going to the ISP modem. For cable Internet, RG-6 coaxial cable is needed, not RG-59. Check for t.v. line splitters and remove them as they can introduce noise on the line and lower the signal going to the ISP modem. Connecting to the ISP modem could result in a false positive as the signal to the modem could be just enough to that point then adding on a router, could see problems. The router operation is dependent upon getting good data flow from the ISP modem and the modem is dependent upon getting good signal from the ISP Service.
Router and Wired Configurations
Setup DHCP reserved IP addresses for all devices ON the router. This ensures each devices gets its own IP address when turned on and connected, eliminates IP address conflicts and helps in troubleshooting and maintain consistency for applications that need to connect as well as mapped drives.
Ensure devices are set to auto obtain an IP address.
If http://en.wikipedia.org/wiki/Ipv6 is an option on the router, select Local Connection Only.
Any 2.4Ghz or 5Ghz cordless house phones or WiFi APs near by?
Any other WiFi routers in the area? Link> Use http://www.metageek.net/support/metageek-software-archives/ to find out. Use v3. How many?
WMM Support must be enabled for single wireless N mode to function.
Router Placement
Forum User - "Well I feel really dumb. After moving the router away from other electronic devices my speeds are back to normal. Just a heads up for anyone experiencing slow speeds, you might want to move it away from other electronics and see if that helps."
3-6' feet minimum safe distance between devices.
Placement on main level floor and central in the building and WELL ventilated is preferred. Not in basements or closets as building materials, or near by electronics devices could interfere or hinder good signal propagation.