I am having the same problem with my DGL-4500 Router.  I just switched to Comcast internet and bought a ARRIS SB6121 Modem.  Wired through the router I get speeds between 80-110 mbps.  I unplug and initially get about 15 and then it tails off from there to between 3 and 8.
I am in California
I was playing around with settings and decided to do a factory reset, as that was suggested multiple times.  I am running the 1.22NA Firmware.  That's what came loaded and it says that is the latest.
Since resetting, I only have my laptop connected (drivers checked) as I want to make sure the problem is not being caused by any other device.  But before the factory reset I was also having problems with the Roku. 
SSID name changed.
Running mixed G and N, but tried single G and it didn't help.
Tried both Auto 20/40Mhz and 20Mhz only.
Used inSSIDer and determined that Channel 1 has the fewest co-channels and overlaps.
Using WPA-Personal. 
No cordless phone.
Sequentially I turned 1) OFF Short GI, 2) ON WLAN Partition, 3) OFF Extra Wireless Protection.  Don't have HT 20/40 Co-existence (at least I couldn't find it). 
Router is sitting out in the open, no other electronic devices near it that are on, and am testing about 6 feet away, so I have strong signal strength and shouldn't have any interference. 
Even turned off the firewall, as that seemed to give the Roku fits before the factory reset.
Any other suggestions?