What Hardware version is your router? Look at sticker under router.
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?
What wireless modes are you using?
2.4Ghz Routers only: Try single mode G or N or mixed G and N?
Channel Width set for Auto 20/40Mhz or try 20Mhz only.
Try setting a manual Channel to a open or unused channel. 1, 6 or 11. 11 for single mode N if the channel is clear. 13 for EU regions.
http://kb.linksys.com/Linksys/ukp.aspx?vw=1&docid=f2625b15a5d7454b8e7fafbe65d5aa63_4009.xml&pid=80&respid=0&snid=4&dispid=0&cpage=search
What security mode are you using? Preferred security is WPA-Personal. WPA2/AES Only. Some WiFi adapters don't support AES, so you might want to try TPIK [/url] only or Auto TPIK and AES.
What wireless devices do you have connected?
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.